IRLF 


B    3    E71    T?fl 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 


PRESENTED  BY 

PROF.  CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 


Microscopical  Praxis. 


PHOTOMICROGRAPH 

OF  THE 
TEST  PODURA    (LEPIDOCYRTIS   CURVICOLLIS^  X  3OOO. 


H.  Q.  PIFFARD,  M.D.,  NEW  YORK. 
Spencer  &  Smith,  Obj.  1/15.     N.  A.  1.35. 


Microscopical  Praxis 


OR 


Simple  Methods  of  Ascertaining  the 
Properties  of  Various  Micro- 
scopical Accessories  * 


BY 


ALFRED  C.  STOKES,  M.  D., 


AUTHOR  OF    "A  CONTRIBUTION  TOWARD  A  HISTORY  OF  THE  FRESH-WATER  INFUSORIA 
OF  THE  UNITED  STATES;"    "A  KEY  TO  THE  GENERA  AND  SPECIES  OF  THE  FRESH- 
WATER ALGA:  AND  DESMIDIE^:  OF  THE  UNITED  STATES;"    "MICROSCOPY 
FOR  BEGINNERS;"    "OBJECTS  FOR  THE  MICROSCOPE,"  ETC. 


ILLUSTRATED 


PORTLAND,    CONN.; 

EDWARD  F.   BIGELOW 

1894 


COPYRIGHT,  1894, 

BY 
EDWARD  F.  BIGELOW. 


• 


By  the  Author  of  "  Microscopical  Praxis." 

I. 

MICROSCOPY  FOR  BEGINNERS. 
Price,  postpaid,  $1.50. 

II. 
FRESH-WATER  ALGJE  AND  THE  DES- 


Price,  postpaid,  $1.25. 
Address  E.   F.   Bigelow,  Portland,  Conn. 


M351791 


MICROSCOPICAL    PRAXIS.  XI 


Contents. 


PAGE 

Illustrations,      -  xii 

Preface,     -  xiii 

The  Pocket-Lens,  i 

The  Microscope  Stand  and  its  Parts,  7 

The  Micrometer,  58 

How  to  care  for  and  use  the  Microscope,  67 

Thin  cover-glass,  81 

The  Objective,  90 

Refractive  Index  of  Immersion  Fluids,  164 

Finders  and  similar  Devices,     -  168 

Sub-stage  Illuminators,      -  169 

The  sub-stage  Condenser,  169 

Black-ground  Illuminators,  181 

The  Paraboloid,  182 

The  Spot-Lens,  183 

The  Woodward  Prism,  184 

The  Hemispherical  Lens,    -  186 

Supra-stage  Illuminators,  187 

The  Lieberkuhn,  187 

The  Parabolic  Speculum,    -  *iS8 

The  Vertical  Illuminator,    -  189 

The  Amplifter,   -  193 

The  Erector,  194 

The  Polariscope,  196 

Drawing  the  Object,  200 

Reflectors  and  Camera  Lucidas,  "201 

Live  Boxes,  Growing-Cells  and  other  Accessories, 

^Index, 


Xll  MICROSCOPICAL    PRAXIS. 


Illustrations. 


PAGE 

Frontispiece. 

Fig.     i.  Stewart's  Safety  Stage,  28 
Fig.    2.  Pennock's   Method  of  Centring  the  Il- 
lumination, 44 
Fig.    3.  Bullock's  Apparatus  for  measuring  the 

Power  of  Eye-pieces.  54 

Figs.  4-9.  The  Podura  Scale,  97 

Fig.  10.  Tube-length  used  by  various  Opticians,  102 
Fig.  ii.  H.  L.  Smith's  Test-slide  for  refractive 

Index,       -  165 
Fig.  12.  Illumination  of  opaque  Objects  by  the 

Woodward  Prism,     -  185 

Fig.  13.  The  Vertical  Illuminator,  192 

Fig.  14.  Explanation  of  Polarised  Light,  -  197 
Fig.  15.  Mrs.   S.   H.   Gage's   Drawing-desk   for 

the  Abbe  Camera  Lucida,  207 

Fig.  16.  E.  A.  Apgar's  Mechanical  Finger,  210 
Figs. '17,  18,  19.  Simple  Life-slides,              215,  217,  219 

Fig.  20.  Beale's  Growing-cell,  221 

Fig.  21.  H.  L.  Smith's  Growing-cell,  222 

Fig.  22.  Sternberg's  Culture-cell,  224 

Fig.  23.  Pagan's  Life-slide,  226 

The  Acme  Lamp,      -  31 


Preface. 


The  writer  was  ^  recently  asked:  How  is  the  value  of 
the  spaces  on  the  eye-piece  micrometer  ascertained? 
What  is  done  with  the  stage-micrometer  when  the  eye- 
piece micrometer  is  in  use  ?  And,  How  can  I  learn  the 
magnifying  power  of  a  simple  microscope  ? 

It  was  these  and  similar  questions  which  lead  to  the 
preparing  of  this  unpretentious  little  book  for  the  use 
of  those  beginning  microscopists  that  are  desirous  of 
knowing  how  to  ascertain  the  matters  referred  to  in 
these  queries,  and  such  other  points  that  are  generally 
unknown  by  the  novice  in  connection  with  the  various 
microscopical  accessories. 

There  seems  to  be  an  actual  need  of  a  book  of  simple 
explanations  of  such  processes  as  the  ascertaining  of 
initial  magnifying  power  of  microscope-objectives,  of 
the  focus  of  the  concave  mirror,  of  the  power  of  the 
eye-piece,  the  measuring  of  the  actual  and  of  the  appar- 
ent field  of  view,  the  measuring  of  the  thickness  of  thin 
cover-glass,  the  measuring  of  the  refractive  index  of 
immersion-fluids,  and  other  matters  of  the  kind  which 
every  amateur  microscopist  sooner  or  later  is  anxious 
to  know,  and  which  the  books  and  the  magazines  either 


XIV  MICROSCOPICAL    PRAXIS. 

entirely  neglect,  or  publish  in  such  a  manner  as  to  be 
inaccessible  to  the  general  reader. 

It  is  in  the  explaining  of  these  uncomplicated  but  im- 
portant methods  in  the  use  of  the  microscope  that  the 
book  has  its  excuse  for  being,  if  it  have  any.  Its  author 
has  intended  that  the  inquirer  should  find  in  its  pages, 
in  the  form  of  a  few  simply-expressed  directions,  an  an- 
swer to  his  question,  instead  of  writing  to  some  micro- 
scopical magazine  to  ask,  for  instance,  How  shall  I  as- 
certain the  focal  length  of  my  pocket-lens  ?  and  then 
waiting  several  months  for  some  leisurely  microscopist 
to  reply,  and  perhaps  waiting  in  vain. 

That  the  book  contains  all  the  points  that  might 
justly  be  expected  to  be  found  in  it,  the  writer  can- 
scarcely  hope;  he  has  not  attempted  to  make  a  micro- 
scopical encyclopaedia,  but  only  an  elementary  praxis 
of  those  common  things  which,  at  the  beginning  of  his 
own  use  of  the  microscope,  he  would  have  liked  to 
know,  but  could  not  learn,  because  the  sources  of  in- 
formation were  then  beyond  his  reach.  For  the  use  of 
all  amateur  microscopists  in  a  similarly  questioning 
mood,  the  book  is  submitted  with  the  author's  good  in- 
tentions. 

An  attempt  has  been  made  to  give  credit  to  all  those 
microscopists  whose  methods  have  been  referred  to, 
but  as  some  of  these  ways  and  means  are  so  generally 
current  that  they  have  become  common  property  and 
their  sources  have  been  forgotten,  it  has  not  been  pos- 
sible in  all  cases  to  discover  to  whom  the  credit  be- 
longs. If  the  reader  should  happen  to  find  some  of  his 
own  original  and  cherished  methods  described  with- 
out the  mention  of  his  name,  he  may  feel  sure  that  the 
omission  is  due,  not  to  the  writer's  intentional  lack  of 
courtesy,  but  solely  to  his  ignorance. 


MICROSCOPICAL    PRAXIS.  XV 

Many,  perhaps  the  majority,  of  the  means  here  sug- 
gested to  certain  ends  are  not  scientifically  and  mathe- 
matically accurate,  and  no  such  claim  is  made  for  them; 
yet  they  are  intended  to  be  sufficiently  correct,  and  the 
results  sufficiently  close,  for  every  practical  purpose  of 
the  non-professional  microscopist,  for  whom  they  have 
been  prepared  and  with  whom  they  are  now  left. 

TRENTON,  N.  J.,  1894. 


Microscopical  Praxis. 


The  Pocket-Lens. 

The  pocket-lens  is  simply  a  double-convex  lens  which 
is  usually  mounted  in  a  vulcanite  frame.  There  are 
several  varieties  in  the-market,  but  however  they  may 
vary  in  magnifying  power  and  in  appearance,  they  are 
all  essentially  simple  double-convex  lenses.  Each  may 
consist  of  a  single  lens,  or  of  several  so  mounted  that  one 
or  all  may  be  used  at  a  time,  the  powers  varying  ac- 
cording to  the  combination.  With  the  single  lens  the 
magnifying  power  always  remains  the  same.  This  is 
the  preferable  form.  It  usually  magnifies  sufficiently 
for  all  practical  purposes  and  it  is  easily  and  rapidly 
used;  while  with  the  combination  form  the  power  is 
often  inconveniently  great  and  the  field  of  view  corres- 
pondingly limited.  The  working-distance,  too,  or  that 
distance  between  the  lens  and  the  object  when  in  focus, 
is  also  much  shorter  than  with  the  single,  low-power 
pocket-lens. 

Three  simple  double-convex  lenses  usually  form  the 
combination  pocket-lens,  each  giving  a  different  mag- 
nifying power,  the  highest  being  in  use  when  the  three 
are  employed  together,  the  combination  then  acting  as 
does  the  single  lens.  In  such  cases  there  is  usually  a 
diaphragm  inserted  between  the  highest-power  glass 
and  the  next  powerful,  in  order  to  obstruct  the  passage 


2  MICROSCOPICAL    PRAXIS. 

of  the  rays  of  light  that  pass  through  the  margin  of  the 
lens,  and  which  do  not  come  to  the  same  focus  as  those 
that  pass  through  the  centre.  This  spherical  aberra- 
tion, as  it  is  called,  being  especially  noticeable  with 
high-power  pocket-lenses.  It  exists  in  all  forms,  ex- 
cept in  the  achromatic  triplet,  in  which  it  is  corrected 
by  the  use  of  a  combination  of  lenses  cemented  in  pro- 
per position  by  the  optician. 

The  more  the  lens  magnifies  the  closer  it  must  be 
brought  to  the  object  in  order  to  focus  it.  These  com- 
bination lenses  are  therefore  not  so  pleasant  to  use  as 
are  the  single  forms.  When  employing  the  entire  com- 
bination the  highest-power  lens  should  be  nearest  the 
object  to  be  examined,  while  either  surface  of  the 
single  lens  may  be  toward  the  object. 

A  single  lens  magnifying  about  five  diameters  and 
having  about  two  inches  of  focal  distance  is  an  excel- 
lent one  for  ordinary  use.  If  an  achromatic  triplet 
should  be  desired,  and  it  is  an  admirable  thing  to 
possess  although  rather  expensive,  one  with  the  half- 
inch  focus  should  be  selected,  as  it  will  then  magnify 
about  twenty  diameters  and  be  amply  sufficient  for 
all  kinds  of  field-work. 


To  Measure  the  Focus  of  the  Pocket-Lens. 

To  ascertain  the  focal  distance  of  the  lens  by  day- 
light, focus  the  bar  of  a  window  on  a  distant  wall,  the 


MICROSCOPICAL    PRAXIS.  3 

distance  from  the  window  being  as  great  as  possible,  at 
least  the  width  of  the  room.  Move  the  lens  to  and  fro 
before  the  wall  opposite  the  window  until  the  bar,  the 
curtain-fringe  or  a  tassel  is  sharply  in  focus  on  the 
white  surface.  Then  measure  the  distance  of  the  lens 
from  the  wall,  and  that  distance  will  be  focal  length. 
The  image  will  be  small,  and  it  should  be  perfectly 
sharp  and  distinct. 

At  night  remove  the  lamp  as  far  as  possible  from 
the  wall  and  use  the  edge  of  the  flame.  In  this  case 
the  focal  point  will  be  represented  by  a  minute  spot  of 
light  of  great  brilliancy,  and  the  experimenter  should 
be  sure  that  the  spot  is  as  small  as  possible,  and  that 
there  is  not  an  indistinct  haze  or  halo  around  its  mar- 
gin. Measure  the  distance  as  in  the  other  experiment. 
In  both  cases  take  care  to  have  the  lens  parallel  with 
the  wall.  If  it  is  held  obliquely,  the  spot  of  light  will 
be  irregular  in  form  and  indistinctly  defined.  This  is  es- 
pecially to  be  remembered  in  the  evening,  when  the 
little  spot  should  be  perfectly  circular,  and  intensely 
bright. 


To    Ascertain    the    Magnifying    Power    of    the 
Pocket-Lens. 

The  opticians  have  found  it  necessary  to  select  an 
arbitrary  distance  at  which  vision  with  the  naked  eye 
shall  be  measured,  a  ten-inch  space  having  been  agreed 


4  MICROSCOPICAL    PRAXIS. 

upon.  The  standard  bodies  of  British  and  of  American 
microscopes  are  of  that  length,  and  the  best  objectives 
are  corrected  for  such  tubes.  The  drawing  and  the 
measuring  of  objects  under  the  compound  instrument 
are  commonly  made  at  that  distance  from  the  eye-piece, 
and  the  pocket-lens  comes  in  for  similar  treatment,  and 
for  the  same  arbitrary  reason. 

To  measure  the  magnifying  power  of  this  lens  place 
it  on  a  support  so  that  the  upper  surface  of  the  glass 
shall  be  ten  inches  above  the  table,  and  below  it  spread 
a  sheet  of  white  paper.  Have  at  hand  a  rule  divided  to 
the  tenths  of  an  inch.  Hold  the  rule  at  right  angles 
with  the  microscopist's  body  for  convenience  ot 
manipulation,  that  is,  with  its  width  parallel  with  his 
width.  In  the  right  hand  lift  the  ruler  into  focus  under 
the  lens  and  hold  it  there  steadily,  keeping  the  head 
immovable  in  one  position,  and  with  one  eye,  prefer- 
ably the  left  eye,  closed.  When  the  object  is  in  focus, 
open  both  eyes  and  the  lines  will  appear  to  be  projected 
on  the  paper.  With  a  pair  of  dividers  in  the  left  hand 
ascertain  the  apparent  width  of  one  of  these  spaces  and 
measure  that  distance  on  the  rule,  when  each  tenth-inch 
between  the  divider's  points  will  represent  one  diameter 
in  magnifying  power.  If  the  space  occupies  five-tenths, 
as  it  probably  will  with  a  lens  having  a  two-inch  focus, 
the  magnifying  power  will  be  five  diameters;  that  is, 
the  object  will  be  enlarged  five  diameters  in  length  and 
in  width,  or  twenty-five  times. 

A  lens  of  any  kind  magnifying  ten  diameters  is  said 
to  magnify  one  hundred  times,  or  ten  diameters  in  each 
direction,  "times"  representing  the  square  of  the  "di- 
ameters," and  the  diameters  the  square-root  of  the 
times.  If  this  fact  is  recollected  there  is  no  danger 
of  being  deceived  by  the  ridiculous  claims  for  power 


MICROSCOPICAL    PRAXIS.  5 

made  for  their  lenses  by  some  dealers,  who  are  careful 
to  express  the  amplification  by  "times,"  with  the  inten- 
tion to  lead  astray  the  unsuspecting,  extolling  a  lens, 
for  instance,  because  it  magnifies  "a  thousand  times," 
which  is  only  one  hundred  diameters,  not  a  high  power 
with  the  compound  microscope,  but  utterly  impossible 
with  the  simple  pocket-lens. 


A  Test  for  the  Pocket-Lens. 

Mr.  Julien  Deby,  who  has  suggested  the  following 
test  for  the  excellence  of  a  pocket-lens,  remarks,  that 
although  many  objects  are  accessible  for  the  determi- 
nation of  the  merit  of  the  medium-power  and  of  the 
low-power  magnifying  glasses  of  the  compound  micro- 
scope, none  is  on  record  for  that  most  useful  instru- 
ment to  the  naturalist,  the  hand-lens.  This  needed 
test  he  considers  to  be  the  elytron,  or  wing-cover,  of 
the  aquatic  beetle  Gyrinus  marinus.  The  lens  should 
show  not  only  the  longitudinal  rows  of  large  dots,  but 
also  the  fine  intermediate  punctations,  the  elytra  of  the 
male  beetles  of  the  genus  being  more  difficult  of  reso- 
lution, since  the  markings  are  there  finer  than  those  on 
the  wing-cases  of  the  females.  But  unfortunately  the 
species  of  "whirligig"  beetle  selected  by  Mr.  Deby  is 
not  found  in  this  country.  Other  forms  are  here  how- 
ever, and  as  all  the  species  so  closely  resemble  one  an- 
other that  entomologists  differ  in  their  opinion  as  to 


6  MICROSCOPICAL    PRAXIS. 

what  constitutes  a  distinct  species,  it  is  probable  that 
the  wing-cases  of  almost  any  form  will  answer  the  pur- 
poses of  a  test. 

To  enable  the  microscopist  to  identify  the  forms  that 
he  may  find,  and  whose  elytra  he  may  experiment  with 
as  a  test  for  his  pocket-lens,  the  following  key  to  the 
species  is  given.  It  was  originally  published  in  "The 
Journal  of  Microscopy,"  and  is  reproduced  here  some- 
what changed  in  form. 


Key    to   the    Species    of    the    aquatic    Beetle 
Gyrinus, 

A.  Underside  entirely  rusty-red  (a). 

B.  Underside  wholly  or  chiefly  black,  legs  reddish  (b\ 
a.  Punctures   on  elytra  scarcely   feebler  toward  the 

suture,  G.  ininutus,  Fab. 

a.  Punctures  on   elytra  finer   toward  the  suture,    G. 

ur ina tor,  111. 

b.  Reflexed  margin  of  thorax   and   elytra  reddish  (c]. 
.  b.  Reflexed  margin  brassy  black  (/). 

e.  Body  ovate  or  oval  (d). 

c.  Body  elongate-oblong  with  nearly  parallel  sides,  G. 

bicolor,  Payte. 

c.  Body  oblong-ovate  (e). 

J.  Punctures  on  elytra  distinctly   finer  toward  the  su- 
ture.     G.  natator,  Sch. 

tl.   Punctures  scarcely  finer,  G.  stiff riani,  Scrip. 


MICROSCOPICAL    PRAXIS.  7 

c.  Interstices  on  elytra  impunctate,  G.  distinctus,  Hub. 
e.  Interstices  indistinctly  punctured,  G.  caspius,  Men. 

e.  Interstices    closely  and   distinctly    punctured,    G. 

colymbuS)  Fr. 

f.  Punctures  on  elytra  scarcely  finer  toward  the  su- 

ture, G.  marinus,  Gyll. 

f.  Punctures  much  finer  toward  the  suture,  G.  opacus, 
Sahib. 

Our  well-known  species  is  common  and  abundant  on 
the  surface  of  still  waters  in  the  spring  and  the  summer, 
floating  together  in  companies,  or  swimming  rapidly 
singly  and  often  in  circles,  their  vivacity  making  them 
rather  difficult  to  capture  without  a  net.  They  have' a 
not  unpleasant  musk-like  odor  when  held  in  the  closed 
hand.  They  are  everywhere  known  as  the  "whirligig 
beetles." 


The  Stand  and  its  Parts. 

The  compound  microscope  includes  the  eye-piece 
and  the  objective,  with  the  brass  parts  which  support 
the  mirror  and  the  optical  portions;  that  is,  it  includes 
the  entire  instrument  as  prepared  for  the  examination 
of  objects,  while  the  stand  is  only  the  brass  part  with 
the  mirror  and  usually  the  eye-piece.  To  convert  the 


8  MICROSCOPICAL    PRAXIS. 

stand  into  a  microscope  the  addition  of  an  objective  is 
essential.  It  is  well  to  remember  the  distinction  be- 
tween these  terms,  especially  in  talking  with  the  deal- 
ers. If  you  ask  a  dealer  for  a  microscope,  he  will  antic- 
ipate the  sale  of  not  only  the  stand  but  of  a  series  of 
objectives  as  well;  but  it  is  often  to  the  purchaser's  ad- 
vantage to  buy  a  stand  of  one  manufacturer  and  the 
objectives  from  another.  The  following  is  a  list  of  the 
parts  of  the  stand,  with  the  definition  of  each. 

FOOT  OR  BASE: — The  part  on  which  the  entire  instru- 
ment rests,  usually  in  the  form  of  a  low  and  flattened 
tripod. 

PILLARS: — The  upright,  cylindrical  rods  attached  to 
the  centre  of  the  foot  and  supporting  the  portions 
above.  Many  stands  have  only  a  single  pillar. 

ARM: — The  part,  usually  curved,  attached  to  the  pil- 
lars by  means  of  a  joint,  so  that  the  working-portions 
may  be  inclined  at  the  convenience  of  the  observer. 
The  upper  frontal  end  of  the  arm  carries  the  body. 

BODY: — The  movable  tube  to  which  are  attached  the 
magnifying  or  optical  parts. 

DRAW-TUBE: — An  additional,  movable  tube  within 
the  body,  whence  it  may  be  withdrawn  at  the  will  of  the 
microscopist  to  increase  the  magnifying  power,  or  for 
other  purposes. 

EYE-PIECE  OR  OCULAR: — The  short  tube  containing 
an  upper  and  a  lower  lens,  and  fitting  loosely  into  the 
upper  end  of  the  body,  or  of  the  draw-tube.  It  is  so 
called  because  it  is  near  the  observer's  eye  when  the 
microscope  is  in  use. 

OBJECTIVES: — The  compound  magnifying  lens  applied 
to  the  lower  end  of  the  body.  It  is  so  named  because 
it  is  near  the  object  to  be  examined  when  the  microscop- 
ist is  working  with  the  instrument. 


MICROSCOPICAL    PRAXIS.  9 

Focus:— The  position  of  the  lenses  in  which  the  ob- 
ject is  seen  most  distinctly.  Seeking  this  point  by 
moving  the  lens  closer  to  the  object  or  further  away,  is 
called  focussing. 

FIELD  OF  VIEW: — The  circular,  lighted  space  seen  by 
looking  through  the  microscope.  The  object  to  be  ex- 
amined, when  it  is  in  this  space,  is  said  to  be  in  the 
field. 

COARSE  ADJUSTMENT: — The  means  by  which  the 
body  is  moved  up  and  down  rapidly,  and  the  objective 
brought  into  approximate  focus.  In  the  cheaper  stands 
this  is  accomplished  by  sliding  the  body  through  a  col- 
lar which  is  immovably  attached  to  the  arm.  In  other 
and  better  grades  it  is  by  rack  and  pinion. 

RACK  AND  PINION: — The  rack  is  the  straight,  narrow 
piece  of  metal  at  the  back  of  the  body,  with  cogs  or 
teeth  on  its  edge.  The  PINION  is  the  toothed  wheel  on 
the  arm;  it  lifts  the  body  by  its  rotation  and  by  the  ac- 
tion of  its  "leaves"  on  the  rack.  It  is  usually  not  visi- 
ble unless  the  body  is  taken  off  the  arm. 

FINE  ADJUSTMENT: — The  slow  movement  which 
brings  the  objective  exactly  into  focus.  It  is  accom- 
plished by  the  action  of  a  fine-threaded  screw,  at  the 
back  of  the  arm  in  the  better  class  of  modern  instru- 
ments, or  at  the  lower  end  of  the  body  in  the  older  and 
less  praiseworthy  stands.  It  is  also  sometimes  under 
the  front  of  the  arm,  or  even  attached  to  the  stage  and 
moving  it. 

STAGE: — The  thin,  circular  or  oblong  plate  of  metal 
or  of  glass  attached  to  the  lower,  frontal  end  of  the 
arm,  and  used  to  support  the  object  to  be  studied.  It 
is  pierced  by  a  central  opening  for  the  passage  of  the 
light  that  illuminates  the  object. 


IO  MICROSCOPICAL    PRAXIS. 

SPRING  CLIPS: — The  two,  narrow,  curved  strips  of 
metal  attached  to  the  back  part  of  the  upper  surface  of 
the  stage,  and  which  hold  in  place  the  object-carrier  or 
the  glass  slip. 

SLIP: — The  strip  of  glass,  usually  three  inches  long 
by  one  inch  wide,  on  which  the  object  to  be  examined 
is  "mounted."  It  is  placed  under  the  spring  clips, 
when  they  are  present,  as  they  are  not  on  all  stands. 
It  is  held  loosely  in  position  and  is  easily  moved  to  and 
fro  under  the  objective  by  the  fingers,  or  by  movement 
of  the  whole  stage  when  the  spring  clips  are  absent. 

SLIDE: — The  slip  with  the  object  prepared  for  exami- 
nation. The  addition  of  the  object  changes  the  slip 
into  the  slide.  The  object  is  usually  covered  with  a 
square  or  a  circular  piece  of  thin  glass  made  for  the 
purpose. 

DIAPHRAGM: — The  circular,  rotating  disk  of  metal 
pierced  by  openings  of  various  sizes  and  placed  beneath 
the  stage.  It  is  used  to  modify  the  light,  its  different 
apertures  Joeing  turned  below  the  stage-opening  for 
that  purpose.  In  microscopical  literature  and  conver- 
sation any  disk  with  one  or  more  apertures  is  a  dia- 
phragm. Such  disks  are  to  be  found  in  the  body-tube, 
in  the  eye-piece  and  in  some  other  parts  of  the  optical 
accessories. 

STOP: — A  metal  disk  with  a  central,  circular  piece  to 
obstruct  the  passage  of  light,  and  supported  by  narrow, 
radiating  arms;  or  a  metal  disk  with  a  semi-circular  or 
rectangular  portion  cut  from  one  side,  and  used  to  ob- 
struct all  light  except  that  which  passes  obliquely 
through  the  opening. 

MIRROR: — The  silvered  glass  that  reflects  the  light 
through  the  object  or  upon  it,  and  through  the  object- 
ive, the  body  and  the  eye-piece.  It  is  attached  movably 


MICROSCOPICAL    PRAXIS.  II 

to  an  arm  at  the  back  of  the  stage  by  means  of  the  mir- 
ror bar,  and  has  independent  movements  of  its  own.  It 
should  have  both  plane  and  concave  surfaces  in  the 
same  mounting. 

SUB-STAGE: — Those  parts,  except  the  mirror,  which 
are  below  the  stage  and  intended  to  carry  various  ac- 
cessories to  affect  the  illumination.  In  the  best  stands 
the  sub-stage  is  attached  to  the  arm  by  a  movable  sub- 
stage  bar;  in  others  it  is  borne  on  the  mirror-bar, 
while  in  the  cheaper  forms  it  is  attached  to  the  under 
surface  of  the  stage.  From  the  lowest  priced  instru- 
ments it  is  usually  absent,  yet  it  should  always  be  pres- 
ent in  some  form,  as  it  is  of  great  importance  in  many 
kinds  of  work,  since  certain  essential  accessories  can- 
not be  used  without  it. 

MILLED  HEADS: — The  disks,  roughened  or  milled  on 
the  edges  to  give  the  fingers  a  firm  hold,  and  attached 
to  the  fine-adjustment  screw,  to  the  pinion  of  the 
coarse  adjustment,  and  to  the  various  sub-stage  appa- 
ratus of  first-class  stands.  • 


The  Body-Tube. 

Many  stands  with  the  body-tube  about  six  inches 
long  have  a  draw-tube  by  which  to  increase  it  to  the 
standard  length  of  ten  inches.  Most  instruments  of 
this  kind  have  the  short  body  lined  with  cloth,  an  ar- 


12  MICROSCOPICAL    PRAXIS. 

rangement  that  for  a  time  ensures  a  smooth  and  easy 
movement  of  the  draw-tube.  Soon  however,  espec- 
ially if  much  used,  the  latter  begins  to  move  less 
smoothly,  until  finally  it  may  demand  considerable 
muscular  effort  and  both  hands.  The  difficulty  is 
caused  by  the  roughening  of  the  cloth  lining,  which 
must  be  remedied  before  the  tube  can  again  be  moved 
easily.  To  do  this,  take  out  the  draw-tube  and  heat  it 
until  it  is  so  hot  that  it  cannot  be  held  without  some 
discomfort,  and  gently  force  it  into  the  body  where  it 
should  remain  until  cold.  This  in  effect  irons  the  lin- 
ing, which  the  experimenter  must  be  careful  not  to  burn. 
If  one  ironing  is  not  sufficient  the  heating  should  be  re- 
peated. 

It  not  rarely  happens,  too,  that  after  the  cloth-lined 
tube  has  been  used  for  some  time,  the  draw-tube  will 
slowly  slip  downward,  urged  forward  by  its  own  weight 
alone.  The  object  will  gradually  become  dimmer  un- 
til it  will  slowly  fade  away,  unless  it  is  followed  up  by 
a  change  of  focus,  or  unless  the  draw-tube  is  extended. 
This  is  even  a  greater  annoyance  than  to  have  the 
cloth  lining  of  the  body  too  thick  to  admit  the  draw- 
tube,  as  it  continually  interferes  with  the  appearance  of 
the  image.  To  correct  it,  loosen  the  texture  of  the 
cloth  by  the  fingers  alone,  or  by  teasing  it  out  gently 
with  the  forceps. 

The  inside  diameter  of  the  opening  in  the  lower  end 
of  the  body  and  the  outside  diameter  of  the  screw-end 
of  the  objective  are  about  three-fourths  of  an  inch. 
This  is  the  society  screw,  so  called  because  first  sug- 
gested by  the  Royal  Microscopical  Society  of  London. 
For  objectives  as  commonly  made,  it  is  amply  suffici- 
ent, but  for  a  few  exceptional  ones  it  has  been  found 
too  small.  A  few  opticians  make  a  few  objectives 


MICROSCOPICAL    PRAXIS.  13 

whose  angle  of  aperture  is  so  uselessly  great,  and  the 
diameter  of  their  component  lenses  so  uselessly  large, 
that  a  special  screw  is  demanded  on  the  end  of  the 
body.  This,  at  the  suggestion  of  Dr.  W.  W.  Butter- 
field,  of  Indianapolis,  is  made  about  one  and  one- 
fourth  inches  in  diameter.  It  is  called  after  the  in- 
ventor's name  the  Butterfield  screw,  and  is  to  be  found 
on  first-class  American  stands,  which  have  almost  every 
microscopical  convenience,  being  placed  above  the 
society  screw  so  that,  to  use  it,  a  part  of  the  lower  end 
of  the  body  must  be  removed.  It  is  not  on  the  cheaper 
stands,  and  is  not  needed.  Indeed  it  is  a  worthless 
thing  anywhere.  The  society  screw  is  an  indespensa- 
ble  adjunct  of  every  body-tube,  and  is  found  on  all 
American  and  English  stands,  however  small  and 
cheap  they  may  be.  And  when  the  reader  goes  to  the 
optician  to  select  a  stand,  he  will  find  it  to  his  advan- 
tage to  recollect  the  size  of  the  society  screw  and  its 
position. 

In  this  connection  there*  is  one  rule  to  be  remem- 
bered, and  it  is  without  exception.  It  is  that  any 
body-tube  with  a  screw  on  its  lower  end  of  a  diameter 
less  than  three-fourths  of  an  inch  should  be  rejected 
without  a  moment's  hesitation.  And  any  objective 
with  a  screw  on  its  upper  end  of  a  diameter  less  than 
three-fourths  of  an  inch  should  be  rejected  even  more 
speedily.  Any  stand  or  any  objective  without  the 
society  screw  may  safely  be  set  down  as  a  good  thing 
to  be  avoided,  and  the  reader  may  also  justly  view  with 
suspicion  any  objective  which  must  have  an  adapter  to 
fit  it  to  the  society  screw  of  the  stand. 

In  some  first-class  American  stands  a  useful  contri- 
vance is  applied  to  the  lower  end  of  the  body,  and 
named  'the  safety  nose-piece.'  It  consists  of  a  short 


14  MICROSCOPICAL    PRAXIS. 

tube  sliding  easily  within  the  body,  and  pressed  upon 
by  a  spiral  spring  so  that  when  forced  upward,  the 
pressure  of  the  spring  tends  to  return  it  to  its  former 
position.  Its  use  is  to  protect  high-power  objectives 
and  also  the  object.  High-power  lenses  usually  have  a 
short  working-distance,  so  that  there  is  danger,  when 
focussing,  of  touching  the  front  of  the  lens  against  the 
glass  slip,  a  thing  that  every  careful  microscopist  is  anx- 
ious not  to  do,  since  the  thin-glass  cover  over  the  ob- 
ject may  be  cracked,  or  the  lens  scratched  or  broken, 
a  much  more  serious  matter  than  the  breaking  of  a 
cover-glass.  But  should  such  an  accidental  contact 
occur,  before  injury  can  be  done  the  safety  nose-piece 
will  slide  upward,  at  once  relieving  the  pressure  on  the 
objective  and  calling  the  microscopist's  attention  to  the 
danger.  But  any  microscopist  that  will  focus  an  ob- 
jective while  looking  through  it  deserves  to  injure  it. 

Every  objective,  it  makes  no  difference  what  its 
focal  length  may  be,  should  always  be  focussed  while 
the  microscopist  is  looking  under  it,  not  while  he  is 
looking  through  it.  In  the  last-mentioned  position  the 
finishing  touches  may  be  given  by  the  fine  -adjustment, 
if  necessary,  but  in  all  other  events  the  objective  should 
be  gently  racked  down  toward  the  object  while  the  mi- 
croscopist is  looking  across  the  slide  and  under  the  de- 
scending lens.  In  this  way  he  can  see  how  near  the 
objective  is  to  the  slide,  and  can  guard  it  from  danger. 
Then  while  looking  through  it  he  should  rack  it  upward 
until  it  is  in  accurate  focus,  if  it  is  a  low  or  a  medium 
power,  or  if  a  high-power  the  focussing  should  be  ac- 
complished by  a  few  touches  to  the  fine-adjustment 
screw.  In  no  instance  should  any  objective  be  fo- 
cussed downward,  not  even  the  four  inch,  while  the  ob- 
server is  looking  through  the  instrument. 


MICROSCOPICAL    PRAXIS.  15 

Another  interesting  and  often  useful  device  on  first- 
class  American  stands  and  on  a  few  foreign  ones,  is  the 
scale  and  vernier  on  the  side  of  the  body  and  on  the 
arm  of  the  instrument,  for  the  measurement  of  the 
working-distance  of  objectives.  It  is  never  found  on 
smaller  and  cheaper  stands,  where  it  might  well  be  ap- 
plied without  great  additional  cost,  and  be  used  to  as- 
sist the  beginner  in  focussing  his  objectives,  or  in 
measuring  the  working-distance  of  his  lenses. 


The  Draw-tube. 

On  the  majority  of  stands  a  draw-tube  will  be  found 
within  the  body  if  the  latter  is  of  the  standard  length 
of  ten  inches.  On  those  whose  body  is  shorter  than 
ten  inches,  the  draw-tube  is  used  only  to  make  the  ex- 
tension to  the  standard  length,  and  no  additional  tube 
will  be  on  the  instrument.  The  length  of  the  draw- 
tube  is  usually  almost  that  of  the  body,  so  that  it  will 
lengthen  the  latter  enormously  when  fully  extended, 
and  correspondingly  increase  the  magnifying  power.  Its 
upper  end  commonly  projects  somewhat  beyond  that 
part  of  the  body,  affording  a  means  of  manipulation, 
and  for  carrying  the  eye-piece. 

The  lower  end  bears  a  diaphragm  whose  aperture 
sometimes  contains  the  society  screw,  so  that  very  low- 


l6  MICROSCOPICAL    PRAXIS. 

power  objectives  may  be  placed  there  and  used  while 
entirely  within  the  body.  This  is  often  a  great  conven- 
ience in  the  employment  of  such  objectives  as  the  four, 
or  three  inch,  which  have  such  an  exceedingly  long 
working  distance  that  occasionally  the  body  must  be 
raised  so  high  above  the  stage  that  it  will  run  off  the 
rack  before  the  focus  is  obtained,  but  if  the  objective  is 
on  the  diaphragm  of  the  draw-tube  it  may  be  approxi- 
mately focussed  by  pulling  out  that  tube,  the  focussing 
being  completed  by  the  rack  and  pinion  on  the  body. 
These  low-power  objectives  are  at  times  very  useful  in 
the  study  of  large  objects,  where  not  much  amplifica- 
tion is  desired. 

What  are  styled  'Student's  stands'  often  have  the 
draw-tube  diaphragm  supplied  with  the  society  screw. 
It  is  not  restricted  to  first-class  instruments,  as  it  is 
almost  a  necessity  on  any  stand,  for  it  not  only  may 
carry  the  low  powers,  but  also  the  amplifier,  and  the 
analyser  of  the  polariscope.  As  a  diaphragm  of  some 
kind  must  always  be  in  the  tube,  and  as  the  society 
screw  can  add  but  the  veriest  trifle  to  the  cost  of  the 
stand,  the  purchaser  might  do  well  to  seek  an  instru- 
ment with  this  convenience. 

The  draw-tube  is  also  often  externally  graduated  to 
parts  of  an  inch  or  of  centimeters  or  both,  so  that  the' 
distance  to  which  it  is  extended  may  be  recorded  and  a 
desirable  result  be  reproduced  at  any  future  time.  The 
only  graduation  on  the  cheaper  stands  is  a  single  circle 
engraved  at  the  point  to  which  the  tube  must  be  ex- 
tended to  make  the  body  of  the  standard  length. 

When  the  part  is  to  be  drawn  out,  do  so  with  a  strong 
and  steady  pull,  holding  the  body  firmly  in  position  at 
the  same  time,  otherwise  it  may  be  run  off  the  rack^and 
entirely  off  the  arm.  Do  not  turn  the  draw-tube  from 


MICROSCOPICAL    PRAXIS.  1 7 

side  to  side  while  extending  it.  The  latter  method  puts 
a  severe  strain  on  the  rack  and  pinion,  and  on  the  fine 
adjustment  if  it  be  at  the  back  of  the  arm.  When  it  is 
to  be  pushed  in,  grasp  the  milled  head  of  the  pinion,  or 
hold  the  body  so  that  the  latter  can  not  move,  while  the 
tube  is  slowly  and  steadily  pressed  down.  If  these 
precautions  are  not  taken,  and  the  objective  is  on  the 
body,  it  may  be  forced  against  the  slide,  or  the  air 
compressed  within  the  body  may  throw  out  the  eye- 
piece. 


Increase  of  Magnifying  Power  by  the  Use  of  the 
Draw-tube. 

Every  increase  in  the  length  of  the  body-tube  in- 
creases the  magnifying  power  of  the  objective  by 
increasing  the  distance  between  the  lens  and  the 
eye-piece.  The  following  tabulated  statement  of 
the  increase  in  power  to  be  added  to  the  original  ampli- 
fication for  each  inch  to  which  the  draw-tube  is  ex- 
tended is  only  approximately  correct,  but  it  is  near 
enough  for  all  practical  purposes.  It  was  originally 
calculated  and  published  by  Messrs.  R.  and  J.  Beck  to 
accompany  their  list  of  British  microscopes.  The  re- 
sults will  not  hold  good  with  the  objectives  or  with  the 
oculars  of  all  makers,  since  all  A,  B,  C,  eye-pieces  have 
not  the  same  power. 


i8 


MICROSCOPICAL    PRAXIS. 


Objective. 
Focal  length. 

Add  for  each  inch  of  draw-tube  with 
eye-piece, 

A  (2  in.) 

B(iiin.) 

C  (i  in.) 

4   inches. 

i^  diam. 

3  diam. 

5  diam. 

3 

2 

4       " 

6      " 

2 

4        " 

6       " 

8      " 

Ti      " 

5        "' 

7       " 

12         " 

i 

5        " 

J5      " 

20       ,u 

*        " 

8        " 

14      u 

25          " 

4_         " 

14        " 

24      " 

34      " 

i°     " 

24        « 

42      " 

63      « 

i      " 

18 

35      " 

60      " 

i      " 

50        " 

85      - 

140      " 

1                  U 

60 

100         " 

1  80      " 

A     " 

80 

150       " 

300      " 

The  Coarse  Adjustment. 

This  part  consists  of  the  rack  on  the  body-tube,  the 
pinion  with  its  cogged  wheel  acting  in  the  depressions 
or  teeth  of  the  rack,  and  the  milled  heads  on  each  side 
by  which  it  is  manipulated.  Its  action  is  to  raise  or  lower 
the  body  rapidly  so  that  the  objective  may  be  approx- 
imately focussed,  or  be  lifted  above  the  stage  when  the 
slide  is  to  be  placed  in  position,  so  that  the  two  may  be  in 


MICROSCOPICAL    PRAXIS.  19 

no  danger  of  coming  in  contact,  with  the  possible  in- 
jury of  one  or  of  both. 

The  rack  should  be  as  long  as  possible,  so  that  the 
body's  movements  shall  be  ample  for  all  exigencies. 
A  length  of  from  four  and  one-half  to  five  inches  is 
none  too  great.  And  its  motion  should  be,  as  some  one 
has  said,  "as  smooth  as  oil."  A  coarse-adjustment 
mechanism  that  is  noisy  when  in  action,  that  rattles 
and  gnashes  its  teeth,  or  one  that  makes  the  image 
change  its  position  by  throwing  the  body  out  of  centre, 
should  be  rejected.  The  only  place  for  such  a  thing  is 
a  shelf  in  a  museum  of  microscopical  antiquities.  The 
action  should  be  noiseless,  perfectly  smooth,  and  the 
bearings  so  firmly  in  place,  that  the  microscopist  shall 
have  no  fear  that  a  heavy  objective  may  force  the  body 
to  run  downward  by  its  weight  and  by  the  absence 
of  resistance  in  the  coarse-adjustment  mechanism,  an 
accident  that  has  happened.  This  undue  looseness, 
however,  may  occur  after  constant  and  prolonged  ser- 
vice, and  a  remedy  is  usually  provided  by  the  optician, 
who  places  screws  in  such  a  position  on  the  arm  or 
elsewhere,  that  by  tightening  them  the  pinion-beariqgs 
are  tightened,  and  the  trouble  is  corrected  for  a  time. 
Every  stand,  even  the  best,  is  liable  to  this  annoyance 
in  a  greater  or  lesser  extent. 

Some  of  the  cheaper  stands  have  no  coarse  adjust- 
ment. The  body  is  then  encircled  by  a  collar  through 
which  it  moves  when  actuated  by  the  hand,  the  focus 
being  obtained  by  pulling  and  pushing  on  the  tube. 
This  is  a  very  inconvenient  and  undesirable  arrange- 
ment. It  is  awkward,  since  the  friction  is  often  so 
great  that  the  whole  stand  will  move  out  of  position 
before  the  body  will  budge,  and  frequently,  more 
frequently  than  not,  even  when  the  foot  is  heavy 


2O  MICROSCOPICAL    PRAXIS. 

enough  to  keep  the  instrument  firmly  on  the  table, 
both  hands  are  needed  to  manipulate  the  body.  It  is 
dangerous,  too,  since  under  the  circumstances,  the  body 
has  the  obnoxious  habit  of  suddenly  slipping  further 
than  the  microscopist  intends,  stopping  only  when  it 
crashes  againt  the  slide,  where  it  usually  grinds  and 
and  crunches  cover-glass  and  objective  with  apparently 
fiendish  glee.  A  stand  without  a  coarse  adjustment  by 
rack  and  pinion  is  a  good  stand  to  be  permanently  left 
with  the  optician.  No  fine  microscopical  work  can  be  done 
with  an  instrument  whose  body  slides  through  a  friction 
collar.  That  arrangement  may  be  cheap,  but  it  is  also 
a  torment  and  a  peril. 


The  Fine  Adjustment. 

When  the  objective  has  been  imperfectly  focussed  by 
the  coarse  adjustment,  its  position  must  be  often 
further  changed  until  the  image  becomes  clear  and 
bright,  and  the  outlines  as  distinctly  and  sharply  defined 
as  the  lines  in  the  best  steel-engraving.  This  is  accom- 
plished by  means  of  the  fine-adjustment  screw,  which, 
in  the  older  stands,  will  be  found  on  the  lower  end  of 
the  body,  at  the  front;  on  some  of  the  oldest  models 
and  on  a  few  of  the  newest,  it  will  be  attached  to  the 
stage,  but  in  the  best  of  the  more  recent  it  is  at  the 
back  of  the  arm. 


MICROSCOPICAL    PRAXIS.  21 

If  the  fine-adjustment  screw  is  placed  on  the  nose- 
piece,  the  parts  will  sooner  or  later  work  loose,  and 
then  every  time  the  milled  head  is  touched  the  body- 
tube  will  wabble  and  the  image  dance.  And  the  fine- 
adjustment  screw  is  touched  very  often.  During  an 
observation  the  microscopist  moves  the  stage  with  one 
hand,  and  keeps  the  fingers  of  the  other  on  the  adjust- 
ment screw  continuously,  constantly  altering  the  focus 
slightly,  so  as  to  judge  of  the  structure  of  the  object  by 
the  changes  in  the  appearance  of  the  different  optical 
sections  practically  cut  by  the  objective. 

But  the  most  serious  objection  to  the  older  form,  in 
addition  to  what  has  been  previously  mentioned,  is  that 
every  movement  of  the  fine-adjustment  screw  changed 
the  length  of  the  body  and  altered  the  magnifying 
power,  which  was  therefore  never  the  same  for  two 
successive  moments.  With  the  lower  powers  this  was 
scarcely  observable,  but  with  high  powers  it  became 
almost  conspicuously  assertive.  And  during  the  use 
of  the  micrometer  for  the  measurement  of  the  mi- 
croscopic objects,  it  was  a  menacing  danger,  since  the 
value  of  the  micrometer  spaces  was  changed  with  every 
turn  of  the  fine-adjustment  screw.  With  the  mechan- 
ism in  the  arm,  all  this  annoyance  is  done  away  with, 
since  every  movement  of  the  screw  moves  the  entire 
body  including  the  objective  and  the  eye-piece.  If 
properly  made  it  has  no  lost  motion,  and  no  side 
movement.  It  responds  immediately  to  a  touch  of  the 
finger,  moving  the  body  directly  upward  or  downward 
with  no  lateral  play,  so  that  the  image,  even  under  the 
highest  powers,  does  not  seem  to  change  its  position 
from  side  to  side,  a  fatal  defect  if  present.  The  screw 
is  usually  and  preferably  placed  vertically  at  the  back 
of  the  arm,  within  easy  reach  of  the  fingers  as  the  mi- 


22  MICROSCOPICAL    PRAXIS. 

croscopist's  elbow  rests  on  the  table;  but  some  makers 
place  it  under  the  front  of  the  arm,  at  the  side,  or  even 
at  the  back  but  in  a  horizontal  position  or  almost  at 
right  angles  to  the  optic  axis,  that  imaginary  line 
drawn  through  the  centres  of  the  mirror,  sub-stage, 
stage,  objective,  body  and  eye-piece. 

The  milled  head  of  the  fine-adjustment  screw  on  all 
first  class  stands,  and  on  some  of  the  less  expensive, 
notably  on  Messrs.  J.  W.  Queen  &  Co.'s  Acme  No.  3, 
has  the  upper  surface  graduated  for  the  measurement 
of  the  thin  glass  always  covering  microscopic  objects 
when  permanently  mounted.  This  glass  varies  a  good 
deal  in  thickness,  and  since  its  presence  influences  the 
action  of  the  objective,  it  is  often  important  to  know 
just  what  that  thickness  is.  The  value  of  each 
graduated  space  on  the  wheel  depends  upon  several 
contingencies,  seldom  being  the  same  in  any  two  in- 
struments by  different  makers.  What  that  value  is  the 
dealer  will  tell  the  purchaser  if  asked.  On  my  own 
stand  the  distance  between  two  lines  of  the  graduated 
surface  is  equal  to  an  elevation  or  depression  of  the 
body-tube  for  the  one  one-thousandth  of  an  inch. 


MICROSCOPICAL    PRAXIS.  23 

The  Stage. 

The  stage  should  always  be  firm  and  steady  under 
pressure,  but  the  pressure  should  be  applied  judiciously. 
All  microscope-stages,  even  those  on  the  best  stands  by 
Mr.  Bulloch,  Mr.  Zentmayer  and  others,  will  respond 
to  the  pressure  of  the  thumbs,  and  be  sufficiently  de- 
pressed to  carry  the  object  out  of  the  focus  of  a  medium- 
power  objective.  The  stage  that  responds  the  least  is 
the  best,  but  perfection  in  this  regard  seems  beyond 
our  reach.  Neither  is  it  absolutely  essential.  The 
thumbs  never  press  on  the  stage,  unless  they  are  de- 
lirious. No  heavy  objects  are  placed  there.  No  sane 
microscopist  would  put  a  cobble-stone  on  his  stage. 
The  optician  may  be  trusted  to  give  us  the  best  that 
the  conditions  of  the  problem  will  allow,  and  the  ama- 
teur purchaser  need  never  test  the  stiffness  of  the  stage 
by  the  weight  of  his  arms  and  shoulders  transmitted 
through  his  thumbs.  If  he  does,  he  will  deserve  to  test 
the  weight  of  the  dealer's  arms  and  shoulders  trans- 
mitted through  a  club. 

The  stage  should  be  as  thin  as  is  consistent  with  the 
proper  stiffness  and  steadiness.  This  is  needed  to  al- 
low for  certain  effects  of  illumination.  It  sometimes 
happens  that  an  object  must  be  studied  by  what  is 
called  oblique  light,  that  is,  the  mirror  must  be  so  ar- 
ranged below  and  to  one  side  of  the  object,  that  the  re- 
flected light  shall  impinge  upon  it  obliquely,  and  oc- 
casionally very  great  obliquity  is  needed,  which  can  be 
obtained  only  when  the  stage  is  thin,  since  a  thick  stage 
and  a  consequently  deep  aperture  in  the  centre,  would 
prevent  very  oblique  rays  from  passing  through  to  the 
object.  Many  "Students'"  stands  are  faulty  in  this 
respect,  the  makers  seeming  to  think  that  since  oblique 
light  is  needed  only  in  somewhat  advanced  work,  the 


24  MICROSCOPICAL    PRAXIS. 

beginner  will  not  care  for  it.  But  I  believe  in  offering 
the  beginner  advantages  which  he  may  not  at  first  ap- 
preciate, but  which  he  will  at  last  live  up  to.  The 
aesthetic  craze  of  striving  to  "live  up  to"  a  blue  china 
jug  has  happily  passed,  but  the  effect  of  living  up  to 
one's  privileges  remains,  and  the  effort  should  be  even 
the  beginner's. 

-  On  every  stage  there  should  be  some  kind  of  movable 
plate  on  which  the  slide  shall  be  placed,  the  whole 
moving  easily  under  the  impulse  of  the  fingers.  Many 
stands,  the  majority  of  so-called  Students'  stands,  have 
no  such  plate,  spring  clips  being  substituted,  the  slide 
being  placed  under  them  and  moved  about  by  the  fin- 
gers. This  arrangement  answers  well,  provided  the 
clips  will  themselves  remain  permanently  in  position 
when  the  slide  is  manipulated.  The  fingers  are  soon 
educated  to  perform  the  most  delicate  movements, 
guiding  the  slide  by  the  gentlest  pressure,  and  speedily 
learning  to  keep  even  a  living  and  lively  microscopic 
creature  in  the  field;  but  to  be  able  to  do  these  things, 
the  spring  clips  must  not  press  too  heavily  on  the  slide, 
and  they  must  especially  be  firmly  or  immovably  fixed 
in  their  sockets.  As  a  rule,  they  are  fitted  so  loosely 
into  the  holes  provided  for  them  on  the  stage,  that 
scarcely  more  than  a  breath  is  needed  to  move  them. 
The  result  is,  that  during  the  constant  manipulations 
of  the  slide,  they  are  gradually  urged  more  and  more  to 
one  side  or  the  other,  until  finally  they  strike  the  edge 
of  the  cover-glass,  push  it  out  of  place  and,  it  may  be, 
ruin  the  object.  The  microscopist's  eye  is  engaged  at 
the  ocular,  and  his  attention  is  concentrated  on  the 
image,  so  that  he  cannot  pay  special  heed  to  the  spring 
clips  to  see  that  they  are  not  threatening  his  cover- 
glass.  The  Griffith-Club  stand  is  in  this  respect  a  model 


MICROSCOPICAL    PRAXIS.  25 

instrument.  With  it  the  spring  clips  are  made  after  a 
new  and  entirely  original  form,  so  that  there  is  abso- 
lutely no  danger  that  they  will  turn  and  rend  the  cover. 
If  all  stages  having  spring  clips  could  have  them  after 
Mr.  E.  H.  Griffith's  model  the  benefit  would  be  great. 
But  if  the  dealer  offers  a  stand  with  very  loose  clips, 
reject  it  until  he  remedies  the  defect  by  fastening  them 
in  their  sockets.  Then  manipulations  as  delicate  as  any 
to  be  made  anywhere  may  be  made  with  the  slide  under 
them;  but  a  stand  with  loose  clips  is  a  delusion  and  a 
snare. 

For  special  studies  special  stages  are  made.  Warm 
stages  are  used,  the  warmth  being  produced  by 
heated  air,  electricity,  hot  water  or  by  heated  metal 
plates.  Some  complicated  arrangements  are  described 
which  are  often  interesting  and  amusing,  for  at  times  it 
would  seem  as  if  the  inventors  of  these  queer  devices 
put  down  on  paper  what  they  think  should  be  useful,  if 
somebody  could  make  them  successful.  They  have  dial 
plates  attached,  and  thermometers,  and  spirit  lamps, 
and  electric  batteries,  and  steam  cylinders,  and  boilers 
with  a  multiplicity  of  rubber  tubes,  all  of  which,  with 
many  others  for  cooling  objects,  for  subjecting  them  to 
the  influence  of  gases,  and  for  other  purposes,  are 
doubtless  more  or  less  useful  in  their  particular  depart- 
ments, but  they  need  not  detain  us  now,  as  the  beginner 
will  not  need  them,  nor  the  advanced  microscopist, 
either,  I  imagine. 

One  of  the  most  delightful  of  microscopical  luxuries, 
one  which  in  some  cases  is  an  absolute  necessity,  is  a 
mechanical  stage,  provided  it  is  the  right  kind.  The 
beginner  will  probably  not  buy  a  stand  with  a  mechan- 
ical stage,  though  he  might  do  worse  things,  but  if  he 
should  even  once  perform  any  serious  work  with  that 


26  MICROSCOPICAL    PRAXIS. 

device,  he  will,  I  am  sure,  never  abandon  it  voluntarily. 
A  mechanical  stage  is  a  "thing  of  beauty,"  and  if  well 
made  the  rest  of  that  hackneyed  quotation  is  descrip- 
tive of  it. 

But  what  is  it  ?  Only  a  stage  so  made  that  the 
horizontal  and  the  vertical  motions  are  accomplished  by 
rank  and  pinion.  The  description  is  short,  it  seems  a 
small  matter,  but  the  stage  is  one  of  the  most  impor- 
tant parts  of  the  stand.  An  inconvenient  stage  means 
an  inconvenient  stand.  If  properly  constructed,  a 
mechanical  stage  is  strong,  light,  firm,  durable,  desirable, 
and  unrelinquishable.  If  improperly  made  it  may  be 
strong,  firm,  thin  and  light;  it  will  be  altogether  abom- 
inable. Scarcely  any  part  of  the  stand  sees  so  much 
active  service  as  the  stage,  unless  it  is  the  fine  adjust- 
ment, and  scarcely  any  part  must  approach  perfection 
so  closely  as  the  stage  unless,  again,  it  is  the  fine-ad- 
justment mechanism.  To  have  this  part  loose  and 
wabbling,  with  the  image  dancing  at  every  turn  of  the 
screw,  is  as  bad  as  having  lost  motion  in  the  racks  and 
pinions  of  the  mechanical  stage,  or  to  have  the 
parts  loose  and  rattling,  or  to  have  them  "lift,"  that 
is,  to  be  raised  above  the  general  plane  surface  when- 
ever the  milled  heads  are  turned  to  bring  the  me- 
chanism into  action.  Every  movement  should  be 
smooth,  prompt,  noiseless.  Every  slightest  touch  of 
the  milled  heads  should  be  followed  by  a  movement  of  the 
stage-plate  that  shall  be  visible  through  the  microscope, 
whatever  it  may  be  to  the  naked  eye.  To  be  forced 
to  turn  the  milled  heads  through  part  of  a  revolution 
before  the  leaves  of  the  pinion  engage  the  teeth  of  the 
rack,  is  something  that  will  undermine  the  best  dis- 
position the  microscopist  is  blessed  with; and  to  feel  the 
pinion  crunch  against  the  rack,  while  the  whole  jolts 


MICROSCOPICAL    PRAXIS.  27 

and  bounces  along,  and  lifts  the  object  out  of  focus, 
will  complete  the  ruin.  Nothing  of  that  kind  is  made 
in  the  United  States,  but  something  very  similar  is 
made  outside  of  the  United  States. 

The  beginner,  however,  must  understand  that  a  me- 
chanical stage,  although  one  of  .the  most  desirable  of 
microscopical  luxuries,  is  by  no  means  a  necessity. 
Work  as  good,  important  and  valuable  may  be  done 
without  it  as  with  it.  The  convenience  and  accuracy 
of  its  movements,  its  immediate  response  to  every 
touch,  and  the  confidence  that  the  microscopist 
speedily  learns  to  have  in  it,  are  among  its  attractive 
qualities. 

One  great  advantage  in  the  use  of  the  mechanical 
stage  has  not  been  mentioned.  This  is  the  procedure 
called  "sweeping  the  field,"  or  the  act  of  searching 
every  part  of  the  object  field  by  field,  so  as  to  examine 
every  feature  and  to  bring  every  part  of  a  preparation 
under  the  objective.  With  the  ordinary  stage  as 
moved  by  hand,  it  is  easier  to  fail  to  bring  a  minute 
point  or  minute  object  into  the  field  than  it  is  to  find  it. 
The  stage  may  carry  the  desired  object  in  every  direc- 
tion except  the  right  one,  while  with  the  mechanical  de- 
vice, the  slide  may  be  examined  in  every  part  and  cor- 
ner, with  the  certainty  of  finding  what  is  sought.  If 
one  field  be  carefully  scanned  as  the  stage,  carries  the 
slide  horizontally  under  the  lens,  and  the  specimen  be 
then  moved  forward  to  a  distance  equalling  the  width  of 
the  field  and  again  swept  horizontally,  the  desired  ob- 
ject can  scarcely  fail  to  be  finally  caught. 

Many  practical  microscopists  have  devised  safety 
stages  to  be  used  with  very  high-power  objectives,  so  as 
to  prevent  disaster,  should  the  lens  and  the  cover-glass 
come  in  contact.  They  seem  hardly  necessary  for  the 


28 


MICROSCOPICAL    PRAXIS. 


careful  microscopist,  who  is  usually  so  cautious  in  ap- 
proximating objective  and  slide,  that  contact  rarely 
takes  place. 

But  probably  the  simplest,  and  therefore  the  best, 
among  the  many  safety  stages  described  by  their  in- 
ventors, is  the  one  designed  by  Mr.  C.  Stewart  and 
published  in  the  "JOURNAL  OF  THE  ROYAL  MICROSCOPI- 
CAL SOCIETY"  of.  London,  for  January,  1884.  It  is  in- 
tended, as  Mr.  Stewart  says,  to  provide  an  economical 
but  effective  arrangement  for  protecting  slides  from 
breakage  when  being  exhibited  under  high  powers  to 
large  classes  of  students,  an  intention  which  suggests 
the  usefulness  of  the  device  at  microscopical  soirees. 


-77—} 


FIG.  i.     Stewart's  Safety  Stage. 

It  is  shown  in  Fig.  i.  It  consists  of  a  wooden  strip 
about  as  long  as  the  glass  slide  and  somewhat  wider, 
with  a  central  aperture,  and  two  -wooden  side-pieces 
about  one-quarter  of  an  inch  high.  To  each  of  the  lat- 
ter is  fastened  a  thin  strip  of  brass  which  projects  be- 
yond the  ends  of  the  wooden  pieces.  Across  these 
projecting  ends  two  small  rubber  bands  are  stretched, 
and  rtie  slide,  passing  between  them,  is  delicately  sus- 
pended in  the  air,  so  that  at  the  least  touch  of  the  ob- 
jective it  sinks,  and  warns  the  microscopist  of  the 
danger.  This  effective  device  can  be  made  at  home  by 
any  one,  even  the  least  ingenious. 


MICROSCOPICAL    PRAXIS.  29 

The  Mirror-Bar. 

Most  of  the  smaller  American  instruments  now-a- 
days  have  the  swinging  mirror-bar  which  on  them  is 
not  objectionable,  since  they  commonly  have  no  pro- 
vision for  sub-stage  apparatus,  with  the  exception  of 
the  diaphragm  which  is  usually  attached  to  the  lower 
surface  of  the  stage.  The  mirror  may  therefore  be 
swung  above  the  objective  for  its  illumination,  and  so 
fully  supply  the  place,  in  this  connection  at  least,  of 
the  bull's-eye  condensing  lens.  In  use,  the  mirror-bar 
is  turned  on  its  pivot  until  the  mirror  is  above  the  level 
of  the  stage,  when  the  concave  mirror  is  manipulated 
until  the  light  is  thrown  on  the  object,  the  parts  being 
returned  to  their  former  position  at  a  moment's  notice. 
To  obtain  the  effect  of  oblique  light,  the  mirror  and  its 
bar  are  swung  to  one  side  below  the  stage  as  far  as 
may  be  desired,  and  the  light  again  arranged  by  alter- 
ing the  position  of  the  reflector. 

As  a  rule  oblique  light  is  used  only  in  the  study  of 
the  striae  of  diatoms,  and  seldom  for  anything  else. 
Commonly  the  light  is  made  as  accurately  central  as  is 
possible,  and  here  is  one  of  the  objectionable  features 
of  this  otherwise  convenient,  swinging  mirror-bar  as 
applied  to  the  cheaper  stands.  It  often  forces  the  stu- 
dent to  work  unsuspectingly  with  oblique  light.  On 
the  best  instruments  a  provision  is  made  for  clamping 
these  swinging  part's  in  any  position  from  "dead  cen- 
tral" to  any  angle  of  obliquity,  and  there  is  no  danger 
of  involuntarily  using  what  is  not  wanted.  With  the 
advanced  microscopist  this  danger  is  not  great,  since 
he  can  tell  at  a  glance,  whether  the  light  is  properly 
centred  or  not.  With  the  beginner,  however,  it  is  dif- 
ferent. His  eye  is  not  sufficiently  educated  to  per- 
ceive whether  he  is  proceeding  correctly  or  not,  and  if 


30  MICROSCOPICAL    PRAXIS. 

the  mirror-bar  has  no  clamping  or  centring  adjust- 
ments, it  is  as  likely  as  not  to  be  turned  aside,  giving 
the  observer  undesirable  shadows  to  look  at,  and  to 
disturb  the  corrections  of  his  non-adjustable  objectives. 
On  the  best  instruments  then,  these  swinging  parts  are 
an  admirable  convenience.  On  "Students' "  stands 
they  may  be  an  annoyance,  unless  the  beginner  is 
warned  in  advance,  and  unless  the  manufacturers 
would  add  some  simple  marks  to  show  when  the  parts 
are  truly  centred,  as  might  easily  be  done  at  no  extra 
cost,  and  little  extra  labor. 


MICROSCOPICAL    PRAXIS. 


31 


The  Mirrors  and  their  Use.* 

When  but  one  mir- 
ror is  supplied  with 
the  stand  it  generally 
is,  and  always  should 
be,  concave.  As  a  rule 
however,  two  mirrors 
are  mounted  on  oppo- 
site sides  of  the  same 
setting,  one  plane,  the 
other  concave.  The 
size  of  the  former  does 
not  much  matter,  but 
the  latter  should  be  as 
large  as  possible,  two 
inches  in  diameter  not 
being  too  small  for  the 
smallest.  The  surface 
should  be  silvered,  the 
ordinary  looking-glass 
amalgam  of  mercury 
and  tin  not  having  a 
power  of  reflection 
equal  to  that  possessed 
by  a  silvered  surface, 
and  presenting  a  gray-  The  Acme  LamP  of  J« w-  Queen  &  Co- 
ish  appearance  to  the  eye.  The  mirror-box  turns  so 
that  either  the  plane  or  concave  surface  may  be  used  at 
will,  and  it  rotates  on  a  stem  connected  directly  with 
the  bar,  or  by  means  of  a  jointed  arm. 

The  proper  lighting  of  the   object,  and   consequently 
of  the  microscope,  is  one  of  the  most  important  accom- 

*In  the  preparation  of  this  chapter  the  author  has  been  greatly  indebted  to 
an  anonymous  paper  in  "  Science  Gossip." 


32  MICROSCOPICAL    PRAXIS. 

plishments  for  the  beginner  to  acquire,  and  he  will 
learn  that  there  is  a  good  deal  of  mathematics  involved 
in  the  study  of  the  concave  mirror.  From  the  plane 
surface  the  light  is  reflected  unchanged,  that  is,  the 
rays  are  reflected  as  they  are  received,  the  angle  of  in- 
cidence being  equal  to  the  angle  of  reflection.  In  cer- 
tain circumstances  impracticable  in  actual  practice,  the 
plane  mirror  may  focus  the  light  of  the  sky.  With  this 
almost  impossible  exception,  the  action  of  the  plane 
surface  is  entirely  different  from  that  of  the  concave 
mirror.  When  the  latter  receives  parallel  rays  it  re- 
flects them  so  that  they  are  forced  to  converge  and  to 
come  to  a  focus,  while  the  divergent  rays  are  variously 
affected  according  to  certain  optical  principles.  For 
the  mathematics  of  the  action,  and  for  the  optical  laws 
governing  them,  the  reader  must  consult  the  treatises 
dealing  with  the  science  of  optics.  Yet  the  novice 
should  know  the  best  position  for  the  lamp  and  for  the 
mirrors,  in  order  to  obtain  the  most  desirable  effects. 

The  concave  mirror  need  not  necessarily  be  placed 
at  such  a  distance  below  the  stage  that  parallel  rays 
will  be  focussed  on  the  object,  although  it  should  be 
movable  in  a  vertical  direction  so  that  that  focus  may 
be  obtained  if  desired.  It*is  often  necessary  to  have 
something  more  of  the  effect  of  diffused  light  by  lower- 
ing the  mirror,  thus  throwing  a  broader  and  less  in- 
tense illumination  on  the  object.  The  beginner  may 
know  when  the  mirror  is  focussed  by  noticing  on  the 
slide  the  reflection  of  the  window-frame  by  day,  or  of 
the  lamp-flame  at  night.  And  since  the  concave  micro- 
scope-mirror is  always  a  part  of  the  inner  surface  of  a 
hollow  sphere,  its  focal  distance  may  be  readily  learned, 
as  well  as  the  size  of  the  sphere  of  which  it  is  a  part. 

To    ascertain   these,  a   simple    experiment  must   be 


MICROSCOPICAL    PRAXIS.  33 

made  with  parallel  rays  of  light.  The  sun  is  so  far 
from  us  that  the  rays  are  already  practically  parallel 
and  no  apparatus  is  needed  in  the  experiment,  but  if  a 
lamp  be  used,  since  its  rays  are  divergent,  it  must  be 
placed  as  far  as  possible  from  the  mirror,  or  the  light 
must  be  passed  through  a  bull's-eye  condensing  lens. 
In  either  case  the  mirror  must  be  placed  directly  oppo- 
site the  source  of  light,  the  lamp-flame,  if  it  be  used, 
being  as  nearly  as  may  be  on  a  level  with  its  centre; 
then  by  moving  a  white  card  back  and  forth  between 
the  two,  seek  that  point  at  which  the  reflected  spot  is 
the  brightest  and  the  most  distinctly  defined,  measure 
the  distance  from  the  card  to  the  mirror,  and  the  prin- 
cipal focus  in  inches  has  been  obtained.  This  distance 
in  every  concave  mirror  is  one-half  the  radius  of  the 
hollow  sphere  of  which  it  is  a  part,  so  that  by  multiply- 
ing the  focus  by  two  the  radius  will  be  known,  and  by 
multiplying  the  radius  by  two  the  diameter  of  the 
sphere  will  be  obtained.  The  mirror  on  my  own  stand 
has  a  principal  focus  of  four  inches,  the  radius  of  the 
sphere  is  therefore  eight  inches,  and  the  diameter  six- 
teen. 

The  angle  at  which  the  microscope  slopes  in  relation 
to  the  table-top;  the  angle  at  which  the  mirror  slopes  in 
relation  to  the  optic  axis  of  the  microscope;  the  dis- 
tance of  the  mirror  from  the  object,  and  the  distance  of 
the  lamp  from  the  mirror,  affect  for  the  better  or  for 
the  worse  the  excellence  and  the  desirability  of  the  re- 
sultant illumination,  every  change  in  any  of  these  posi- 
tions or  distances  having  its  immediate  effect.  For 
every  inclination  of  the  microscope  a  calculation  may 
be  made,  whose  result  will  give  the  proper  inclination 
of  the  mirror,  the  proper  position  of  the  lamp  and  of 
the  bull's-eye  condenser.  The  beginner,  if  he  wish 


34  MICROSCOPICAL    PRAXIS. 

to  enter  on  the  study  of  microscopical  optics,  will  have 
no  trouble  in  finding  accessible  books  for  consultation. 
The  beginner  who  reads  these  chapters  must  accept  the 
somewhat  dogmatic  statements  without  any  very  ex- 
tended explanatory  reasons. 

It  has  been  found  that  about  the  best  position  for 
the  lamp  and  the  mirror  is  such  that  the  parallel 
rays  shall  form  an  angle  of  ninety  degrees  with  the 
axis  of  the  microscope,  and  forty-five  degrees  with  the 
axis  of  the  mirror,  or  that  imaginary  line  drawn  through 
the  centre  of  the  mirror  and  perpendicular  to  its  sur- 
face, and  that  the  flame  shall  be  on  a  level  with  the 
centre  of  the  mirror  and  the  centre  of  the  bull's  eye 
condensing  lens,  or  the  plano-convex  lens  forming  the 
front  of  the  microscope-lamps  supplied  by  the  dealers. 
The  angle  which,  in  this  case,  the  rays  form  with  the 
axis  of  the  instrument  being  ninety  degrees,  one-half  of 
that,  or  forty-five,  is  the  angle  of  incidence,  and  the 
mirror  is  also  inclined  at  an  angle  of  forty-five  degrees 
with  the  axis  of  the  microscope. 

The  principal  focus  of  the  mirror  and  the  angle  of 
incidence  being  known,  the  proper  distance  of  the 
former  from  the  object  may  be  ascertained  by  apply- 
ing the  following  rule:  "Multiply  the  cosine  (to  be 
found  in  any  book  of  mathematical  tables)  of  the  angle 
of  incidence  by  the  principal  focal  distance,  and  the 
product  will  be  the  required  distance  between  the  mir- 
ror and  the  object."  The  mirror  on  my  own  stand  hav- 
ing a  principal  focus  of  four  inches,  the  cosine  of  forty- 
five  degrees  being  0.70711,  the  mirror  should  be  about 
two  and  eight-tenths  inches  from  the  object  in  order  to 
converge  and  to  focus  parallel  rays  upon  it. 

But  we  are  talking  about  the  use  of  parallel  rays 
when  those  from  artificial  light  are  divergent.  How  are 


MICROSCOPICAL    PRAXIS.  35 

they  to  be  made  parallel  ?  By  the  bull's-eye  condenser, 
either  on  a  separate  stand,  as  commonly  supplied  with 
the  instrument  by  the  dealers,  or  on  the  front  of  the 
Acme  or  of  the  Stratton  microscope-lamp.  The  prop- 
erty of  this  plano-convex  lens  is  to  change  diverging 
into  parallel  rays,  or  parallel  into  convergent,  so  that  we 
have  only  to  place  it  between  the  lamp  and  the  mirror, 
at  the  proper  distance  from  each,  and  the  deed  is  done. 

When  the  bull's-eye  lens  is  used  to  illuminate  the  sur- 
face of  an  opaque  object,  the  question  is  often  asked: 
Which  side  should  be  turned  toward  the  light,  the  plane 
or  the  convex?  The  answer  is  that  it  depends  upon 
the  intensity  of  the  illumination  desired.  The  plano- 
convex lens  of  the  bull's-eye  has  two  principal  foci, 
their  distance  being  somewhat  different  according  to 
which  side  is  toward  the  lamp;  with  the  convex  surface 
in  that  position,  the  bright  spot  of  light  at  the  focal 
point  is  surrounded  by  a  broad  disk  of  fainter  illumina- 
tion, but  with  the  plane  surface  toward  the  lamp  the 
focal  distance  is  increased,  the  light  condensed  at  the 
focus  is  somewhat  less  brilliant,  but  the  circle  of  fainter 
illumination  has  almost  disappeared.  To  get  rid  of 
this  weakening  outer  circle  and  to  have,  only  the  bright 
spot,  as  well  as  to  gain  the  convenience  of  the  longer 
focus,  it  is  better  to  place  the  plane  side  toward  the  ob- 
ject; fjut  when  the  parallel  rays  are  to  be  thrown  on  the 
concave  mirror  to  be  thence  converged  to  a  bright 
focus  on  the  subject,  it  is  better  to  place  the  convex 
side  toward  the  object. 

The  difference  in  the  appearance  may  be  observed, 
and  the  distance  of  the  two  foci  measured,  by  experi- 
menting with  a  card  in  a  way  that  needs  no  explanation. 
But  to  obtain  the  best  results,  either  when  using  the 
bull's-eye  with  the  concave  mirror,  or  when  using  it  as  a 


36  MICROSCOPICAL    PRAXIS. 

direct  illuminator  for  opaque  objects,  the  flame  should 
always  be  placed  on  a  level  with  the  centre  of  the  lens, 
and  as  nearly  as  possible  at.  the  focal  point. 

Unless  we  have  a  microscope-lamp  like  the  Stratton 
Illuminator,  or  the  Acme,  or  some  other  special  form, 
it  often  happens  that  we  wish  to  dispense  with  the  use 
of  the  bull's-eye  condenser,  which  is  always  more  or  less 
troublesome,  and  to  take  the  light  directly  from  the 
lamp  with  the  concave  mirror.  Here  the  conditions  are 
changed,  the  rays  of  light  being  divergent.  The  con- 
cave mirror  will  focus  them,  but  the  mathematics  of  the 
process  are  too  abstruse  to  be  entered  into  here.  Still 
the  microscopist  may  readily  find  the  proper  distance 
for  the  lamp  from  the  mirror,  or  for  the  mirror  from  the 
object,  to  obtain  the  best  effects,  provided  he  will  make 
an  easy  calculation,  and  refer  to  some  treatise  on  micro- 
scopical optics  for  the  explanations. 

If  it  is  desired  to  know  the  proper  distance  at  which 
to  place  the  mirror  from  the  lamp,  we  must  know  the 
radius  of  the  mirror  (twice  the  principal  focus),  the 
angle  of  incidence  and  the  distance  of  the  mirror  from 
the  object.  If  we  then  multiply  the  radius  by  the 
cosine  of  the  angle  of  incidence,  and  that  product  by 
the  distance  of  the  mirror  from  the  object,  and  divide 
the  result  by  twice  the  distance  of  the  mirror  from  the 
object,  minus  the  product  of  the  radius  by  the  cosine 
of  the  angle  of  incidence,  the  quotient  will  be  the  dis- 
tance in  inches  for  the  lamp  from  the  centre  of  the 
mirror.  This  seems  complex  when  expressed  in  words, 
but  in  algebraic  formula  it  is  seen  to  be  simple  enough. 
Let  A  represent  the  distance  in  inches  between  the 
lamp  and  the  mirror;  </the  distance  from  the  mirror  to 
the  object;  R  the  radius  of  the  mirror,  and  a  the  angle 
of  incidence,  which  in  this  case  we  will  assume  to  be 


MICROSCOPICAL    PRAXIS.  37 

forty-five  degrees.     Then  the  algebraic  formula  will  be, 

.  _d  R  cos  a 
2d — R  cos  a 

If  the  value  of  the  symbols  be  a,  45°;^,4  inches;  ^?,8 
inches;  we  shall  have 


8— 8  X. 707 

or  about  nine  and  one  fourth  inches  (9.23),  the  position 
of  the  lamp  varying  according  to  the  value  of  the  sym- 
bols. 

It  may  be  more  convenient  to  find  the  proper  dis- 
tance for  the  mirror  from  the  object,  that  of  the  lamp 
and  the  angle  of  incidence  being  known.  In  that 
case  the  formula  will  be, 

A  R  cos  a 


d— 


2A — R  cos  a 


Assuming  the  values  to  be;  A,  10  inches;  R,  8  inches; 
and  #,  45°,  we  have 

d— 


20—  8  X.  707 

or  about  four  inches  (3.95). 

These  superficial  studies  scarcely  touch  the  subject 
of  the  concave  mirror,  and  the  student  who  desires  to 
make  a  detailed  examination  must  go  elsewhere  than 
to  these  chapters. 

The  plane  mirror  is  used  only  for  the  illumination  of 
very  low  power  objectives  and  for  certain  special  pur- 
poses to  be  referred  to  hereafter. 

As  intimated,  the  bull's-eye  lens  is  usually  a  trouble- 
some piece  of  apparatus  to  use  successfully.  It  is 


38  MICROSCOPICAL    PRAXIS. 

focussed  with  some  difficulty,  and  unless  correctly 
focussed  it  is  almost  worthless,  while  it  often  happens 
that  after  the  microscopist  has  labored  to  get  the  awk- 
ward thing  into  proper  position,  an  accidental  touch 
disturbes  it,  and  the  careful  adjustments  must  be 
repeated,  with  much  loss  of  time  and  of  patience.  Yet 
a  bull's-eye  lens  is  essential,  and  the  dealer  often  sup- 
plies one  with  the  stand.  My  advice  to  the  beginner, 
however,  is  to  do  without  the  bull's-eye  in  a  separate 
mounting,  as  supplied,  and  to  substitute  Messrs.  J.  W. 
Queen  &  Co.'s  Acme  lamp,  the  Stratton  Illuminator, 
or  some  similar  form.  In  these  the  bull's-eye  is  at- 
tached to  the  front  of  the  lamp,  the  flame  being  properly 
focussed  before  the  plane  surface,  while  the  whole 
arrangement  is  one  of  the  most  convenient,  useful  and 
manageable  devices  that  microscopists  can  have  on  the 
table,  while  the  cost  is  but  little  more  than  that  of  a 
first-class  bull's-eye  lens. 

When  using  a  concave  mirror,  the  lamp,  under  all 
circumstances,  must  be  so  placed  that  the  flame  is  on  a 
level  with  the  centre  of  the  mirror.  This  is  an  essential 
prerequisite  to  the  attainment  of  the  best  results.  Un- 
der all  circumstances,  too,  the  broad  side  of  the  flame 
must  be  kept  away  from  the  mirror.  The  only  proper 
portion  of  the  flame  ever  to  be  employed  is  the  narrow 
edge.  It  may  seem  strange  that  this  slender  line  of 
light,  as  it  appears  to  be,  should  have  more  intense 
illuminating  power  than  the  whole  wide  front  of  the 
flame,  but  such  is  the  fact,  and  in  all  microscopical  in- 
vestigations it  is  the  only  portion  that  should  be  used. 
It  is  said  to  have  about  eight  times  the  intensity  of  the 
broad  side. 

To  the  reader  the  use  of  daylight  or  of  lamp-light 
may  seem  one  of  personal  preference  only;  but  such  is 


MICROSCOPICAL    PRAXIS.  39 

not  the  fact.  My  belief  is  that  the  majority  of  work- 
ing microscopists  never  employ  daylight  when  they  can 
command  artificial  illumination.  The  latter  may  be  ob- 
tained at  almost  any  time  and  is  entirely  manageable. 
It  may  be  made  into  a  blaze  that  shall  almost  blind  the 
user,  or  it  way  be  subdued  and  softened  until  to  see  it 
is  a  pleasure,  its  result  a  wonder  and  its  effect  beautiful. 

The  field  illuminated  by  the  soft,  white,  steady  glow 
of  a  good  microscopical  lamp  is  not  only  charming,  but 
useful;  and  it  is  beneficial  to  the  eye,  to  which  light  is 
a  tonic  and  a  healthful  stimulus.  The  image  produced 
is  the  best  the  objective  is  capable  of  forming.  It  is 
clear,  sharply  defined,  sparkling  and  satisfactory,  if  all 
the  conditions  are  favorable.  A  better  image  may  be 
obtained  from  an  inferior  objective  and  lamp-light 
properly  employed,  than  from  a  good  objective  and 
diffused  daylight  in  whatever  way  the  combination  may 
be  used.  On  the  eye  the  effect  is  not  injurious  even 
after  hours  of  prolonged  investigation,  provided,  of 
course,  the  light  be  properly  modified.  Only  the  eagle 
can  gaze  directly  at  the  sun,  and  even  this  favorite 
simile  of  the  poets  forces  the  eagle  to  pose  as  a  fraud, 
for  the  bird  protects  its  eye  by  the  nictitating  mem- 
brane, the  third  eyelid,  when  gazing  sunward.  No 
microscopist  will  gaze  at  the  unmodified  light  of  a 
strong  flame  focussed  by  a  concave  mirror,  or  perhaps 
intensified  by  the  sub-stage  condenser,  without  protect- 
ing his  eye  by  some  device  for  reducing  the  blinding 
glare.  But  when  such  modification  is  made,  the  effect 
of  artificial  light  is  not  harmful. 

The  image  formed  by  an  objective  of  a  greater 
amplification  than  that  possessed  by  the  one-half  inch 
or  by  the  four-tenths,  does  not  have  by  daylight  that 
exquisite  sharpness  of  outline  and  brilliancy  of  aspect 


40  MICROSCOPICAL    PRAXIS. 

that  it  will  have  by  artificial  light.  The  general 
appearance  of  things  is  watery  and  washed  out,  if  the 
light  be  taken  from  the  blue  sky,  as  must  commonly  be 
done,  or  almost  destructive  of  the  eye  if  taken  directly 
from  the  sun.  The  only  commendable  light  by  day  is 
that  reflected  from  a  white  cloud,  or  from  a  plane  white 
surface  illuminated  by  the  sun.  Direct  sunlight  is 
never  used  except  at  times  in  photomicrography.  But 
it  is  seldom  that  white  clouds  are  sufficiently  accommo- 
dating to  place  themselves  before  the  window  and  to 
remain  stationary  and  properly  illuminated;  and  the 
plane  white  surface  is  not  often  at  one's  command, 
while  a  lamp  is  always  ready.  If  the  microscopist  can 
make  a  plate  of  plaster-of-Paris,  and  take  the  light 
from  it  when  illuminated  by  sunlight,  the  effect  will  be 
good,  almost  as  good  as  the  effect  of  white  cloud 
illumination,  but  while  the  plaster-of-Paris  plate  in 
diffused  daylight  gives  a  white  field,  it  lacks,  with  high 
powers,  that  soft  intensity  obtainable  from  the  smallest 
flame  properly  managed.  On  a  rainy  day  the  microscop- 
ist would  fare  badly  if  he  had  no  artificial  light  at  his 
command,  and  the  microscopist  that  can  work  with  the 
instrument  at  night  or  not  at  all,  would  fare  as  badly  if 
lamp-light  was  as  harmful  and  worthless  as  some 
writers  seem  to  think  it  is. 

With  daylight  and  no  sub-stage  condenser,  use  the 
concave  mirror;  if  the  stand  have  a  sub-stage  condenser 
then  the  plane  mirror  should  be  employed. 

When  artificial  light  is  used,  the  table  and  the  room 
should  be  only  faintly  illuminated;  there  should  be  no 
side  lights  to  throw  their  reflections  where  they  are  not 
wanted,  and  no  extraneous  light  to  interfere  in  any  way; 
there  should  be  only  the  little  spot  of  intense  bright- 
ness in  the  centre  of  the  object,  if  the  sub-stage  con- 


MICROSCOPICAL    PRAXIS.  41 

denser  be  used,  where  it  will  do  all  that  any  one  will 
want  done,  if  it  be  properly  managed.  My  advice  to 
the  beginner  is  to  use  the  light  of  day  in  his  microscop- 
ical work  as  little  as  possible,  and  only  with  the  lowest 
powers,  but  to  pay  particular  attention  to  the  source  of 
his  artificial  supply,  and  to  its  manipulation. 

Never  take  the  light  from  a  gas-flame.  The  quality 
is  not  commendable,  and  the  incessant  flickering  is  per- 
nicious in  every  way.  Kerosene  oil  is  the  best  illumi- 
nant  that  can  be  used.  The  circular  wick  of  the  Ger- 
man student-lamp  gives  a  powerful  light,  but  the  flame 
of  a  small  flat  wick  is  much  to  be  preferred,  since  its 
edge  is  intense  and  perfectly  steady,  the  latter  being  of 
prime  importance,  as  a  trembling  light  is  one  of  the 
worst  things  that  the  microscopist  can  use.  If  the  be- 
ginner does  not  care  to  buy  a  microscopical  lamp,  he 
will  find  that  a  small  hand-lamp,  such  as  may  be  had 
for  twenty-five  cents,  will  be  all  that  he  will  need,  es- 
pecially if  he  use  a  bull's-eye  lens,  or  a  sub-stage  con- 
denser. 

The  light  should  be  as  white  as  possible.  The  yel- 
low glare  of  the  ordinary  flame  as  seen  through  the  mi- 
croscope is  hot  and  acrid.  It  should  be  cool,  soft  and 
soothing.  To  make  it  so  is  not  difficult.  The  older 
microscopists  were  accustomed  to  produce  the  change 
by  filtering  the  light  through  a  glass  globe  of  water 
colored  faintly  blue  by  sulphate  of  copper  (blue  vitriol,) 
deepening  the  tint  by  the  addition  of  strong  ammonia- 
water,  thus  obtaining  the  so-called  monochromatic  light, 
or  light  of  one  color,  often  referred  to  in  microscopical 
literature.  The  result  was  that  the  blue  water  ex- 
cluded the  yellow  rays,  the  light  being  consequently 
cooled  and  softened  in  appearance.  This,  and  indeed 
all  other  methods  of  accomplishing  the  same  end,  some- 


42  MICROSCOPICAL    PRAXIS. 

what  decreases  the  intensity  of  the  illumination,  but 
enough  will  be  left  for  all  purposes,  and  it  seems  to 
have  the  advantage  of  adding  a  desirable  quality  to  the 
light,  even  when  the  half-inch  wick  is  used  and  turned 
so  low  that  the  flame  rises  only  just  above  the  burner. 
The  device  is  troublesome,  but  the  beginner  may  try  it 
by  making  a  cell  with  parallel  sides  of  glass,  uniting 
the  parts  with  hot  Canada-balsam  or  by  a  cement  made 
by  melting  together  yellow  wax  and  Canada-balsam. 
The  cell  should  be  about  one-eighth  inch  deep  from 
front  to  back,  and  filled  with  a  saturated  solution  of  the 
copper-sulphate  and  ammonia,  the  color  of  which 
should  be  as  nearly  sky-blue  as  possible. 

As  Dr.  W.  H.  Dallinger  has  remarked,  "True  mono- 
chromatic light  really  almost  changes  an  achromatic 
lens  into  an  apochromatic  one;  but  the  great  difficulty 
has  been  hitherto  how  to  produce  monochromatic  light 
which  should  be  absolutely  such,  and  yet  be  within  the 
reach  of  all,  and  under  control  as  to  its  measure  of 
intensity  when  employed  with  high  powers."  The 
ammoniated  solution  of  copper-sulphate  is  not  truly 
monochromatic,  and  experiments  have  frequently  been 
made  to  find  an  easily  prepared  and  easily  managed 
solution  whose  effect  should  approach  more  nearly  the 
desired  result.  Such  a  solution  has  been  obtained  by 
certain  microscopists  in  Europe,  and  highly  rec- 
ommended as  being  practically  monochromatic.  I 
have  used  it  to  a  limited  extent,  but  have  not  observed 
that  the  effect  is  any  better  than  with  the  use  of  the 
properly  colored  glass  as  a  light-modifier.  Perhaps 
with  further  study  and  experimentation  the  solution 
might  produce  more  satisfactory  results.  It  is  used  in 
the  way  already  described  for  the  copper-sulphate  so- 
lution, and  gives  with  artificial  illumination  an  orange 


MICROSCOPICAL    PRAXIS.  43 

light  slightly  tinged  with  green.     It  is  made  as  follows: 
Sulphate  of  copper,  2  ounces  and  i%  drams; 
Bichromate  of  potash,  i  dram  and  2  scruples; 
Sulphuric  acid,  12  minims; 
Water,  6^  ounces. 

Similiar  results  may  be  attained  by  simpler  means. 
If  the  microscopist  must  use  day-light,  it  will  be  of  bet- 
ter quality  if  he  will  fasten  a  pane  of  pale  blue  glass  in 
the  window,  and  receive  the  light  through  it.  At  night 
nothing  mo're  is  necessary  than  to  place  one  or  more 
small  pieces  of  glass  of  a  deeper  blue  above  the  ocular, 
or  in  the  sub-stage  between  the  mirror  and  the  object, 
or  between  the  mirror  .and  the  light.  In  the  latter 
event  a  blue-glass  chimney  is  all  that  is  needed,  if  a 
sub-stage  condenser  be  not  used. 

None  of  these  devices  deprives  the  light  of  those 
qualities  demanded  in  what  has  been  called  "micro- 
scopical gymnastics,"  or  the  use  of  first-class  objec- 
tives to  show  the  finest  striations  on  difficult  diatoms, 
or  a  series  of  lines  closely  ruled  on  glass  plates,  such 
as  those  made  by  the  late  F.  A.  Nobert,  of  Germany, 
and  the  late  Charles  Fasoldt,  of  Albany,  N.  Y.  If  the 
reader  desires  to  practise  microscopical  gymnastics, 
however,  he  will  find  that  the  sulphate  of  copper  or  the 
bichromate  solution  will  be  a  rather  better  assistant 
than  the  blue  glass,  since  the  former  is  said,  by  the  late 
Dr.  J.  J.  Woodward,  to  increase  the  defining  power  of 
first-class  objectives  when  used  in  these  gymnastic  ex- 
ercises, as  the  color  may  be  controlled,  while 
glass  of  just  the  right  sky-blue  shade  is  not  so 
common  as  it  should  be.  For  all  other  purposes  the 
glass  is  amply  sufficient,  supplying  the  best  obtainable 
modification  of  the  illumination. 


44  MICROSCOPICAL    PRAXIS. 

The  centre  is  an  important  point  in  the  microscope. 
The  optician  takes  the  greatest  pains  to  have  the  com- 
ponent lenses  of  his  objectives  most  accurately  centred.. 
The  maker  of  the  stand  is  careful  to  have  all  movable 
parts  act  in  a  certain  relation  to  the  optical  centre  or 
axis  of  the  instrument;  and  the  working  microscopist  is 
always  anxious. to  have  the  illuminating  beam  strictly 
central,  except  when  he  desires  to  use  oblique  light. 
The  centre  of  tne  mirror  is  always  in  che  optical  axis 
of  the  microscope,  or  should  be,  for  ordinary  work. 
To  accomplish  this  centralizing  of  the  illumination  Mr. 
Edward  Pennock  has  suggested  an  admirable  method, 
which  the  beginner  is  advised  to  adopt  for  all  occasions. 
Mr.  Pennock's  instructions  are  substantially  as  follows: 

Having  the  object  in  place  and  lighted  from  the  mir- 
ror, the  objective  screwed  on,  and  the  eye-piece  in  the 
tube,  first  focus.  Then  remove  the  eye-piece  and  ap- 
plying the  eye  centrally  to  the  end  of  the  body-tube, 
notice  the  spot  of  light'at  the  back  of  the  objective.  It 
may  appear  as  in  A,  figure  2,  in  which  case  move  the 
mirror  or  diaphragm  or  both  until  the  illuminating  beam 
appears  central,  as  in  B.  If  now  your  lens  is  a  good 
one,  properly  adjusted,  and  the  inner  circle  of  B  pre- 
sents an  evenly  illuminated  disk,  you  should  obtain  a 


ABC 

Fiji.  -2.  —  Fennock's  method  of  centring  the  illumination. 

good,  sharply  defined  image.  But  there  may  not  be 
light  enough  or  there  may  be  too  much,  in  which  case 
use  a  diaphragm  of  different  size,  or  vary  its  distance 
from  the  object,  thus  varying  the  angular  size  of  the 


MICROSCOPICAL    PRAXIS.  45 

illuminating  pencil.  If  the  diaphragm  be  too  large  or 
placed  too  near  the  object,  it  ceases  to  affect  the  an- 
gular size  of  the  illuminating  beam,  although  it  may  act 
in  another  way,  and  in  this  case  the  image  of  the  mir- 
ror is  seen  within  the  circle  of  the  diaphragm,  as  at  C, 
if  the  objective  is  of  sufficient  aperture. 

If  lamp-light  be  used,  the  image  of  the  flame  may  be 
seen  within  the  disk  of  the  mirror  and  diaphragm,  as  at 
D.  This  shows  that  the  beam  is  not  focussed  upon  the 
object.  This  may  be  remedied  by  a  bull's-eye  con- 
denser placed  near  the  source  of  light,  making  parallel 
the  rays  falling  on  the  mirror,  or  simply  by  altering  the 
distance  of  the  mirror  from  the  stage.  In  some  cases, 
however,  the  mirror  is  not  of  the  right  focus  and  the 
latter  course  cannot  be  adopted.  The  appearance 
should  be  like  B  as  nearly  as  possible.  This  is  as  use- 
ful and  successful  when  the  sub-stage  condenser  is  used 
as  it  is  with  the  mirror  alone. 

Reflected  and  transmitted  light  are  terms  which  are 
somewhat  puzzling  on  first  acquaintance,  although  they 
are  often  met  with  in  microscopical  literature.  They 
refer  to  the  illumination  in  connection  with  the  object, 
and  not,  as  might  be  supposed,  to  the  bull's-eye  con- 
densing lens  or  the  mirror.  A  transparent  substance  is  al- 
ways examined  by  transmitted  light,  the  illumination 
being  passed  or  transmitted  through  it  from  below,  in 
which  event  the  mirror  may  be  used,  or  the  microscope 
may  be  placed  in  a  horizontal  position  and  the  light 
taken  directly  from  the  lamp  flame.  Opaque  objects 
can  not  be  studied  by  transmitted  illumination.  They 
are  restricted  to  reflected  light,  the  illuminating  beams 
being  thrown  upon  the  surface,  from  which  they  are  re- 
flected. Most  objects  seen  by  the  naked  eye  are  seen 
by  reflected  light. 


46  MICROSCOPICAL    PRAXIS. 

The  Diaphragm. 

The  reader  may  suppose  that  there  is  but  one  form 
of  microscope-diaphragm,  a  flat  disk  of  metal  pierced 
near  its  margin  by  apertures  of  various  sizes.  No  idea 
could  be  more  erroneous.  There  are  so  many  forms 
that  a  plausible  supposition  is  that  whenever  an  optician 
has  a  sleepless  night,  instead  of  tossing  on  his  bed  like 
ordinary  folks,  he  invents  a  new  diaphragm.  Then  he 
publishes  it  and  nobody  ever  hears  of  it  again;  cer- 
tainly nobody  ever  uses  one  out  of  a  hundred  of  the 
diaphragmatic  monsters  noted  in  the  books.  There  are 
the  Iris,  and  the  Spiral,  and  the  Cylinder,  and  the  Ca- 
lotte, and  the  Spherical.  There  are  oblong  plates 
pierced  by  all  sorts  of  holes  radiating  in  all  sorts  of  di- 
rections. They  have  horizontal  slits,  and  vertical  slits, 
and  oblique  slits.  They  have  big  holes,  and  little  holes, 
and  pin  holes,  and  round  holes,  and  oblong  holes,  and 
square  holes.  They  are  concave  and  convex.  They 
are  fastened  below  the  stage,  to  its  under  surface,  and 
in  a  depression  that  brings  them  on  a  level  with  the  up- 
per surface.  There  is  one  form  of  two  cylinders  or 
rollers  revolved  by  a  milled  head,  and  encircled  by  two 
conical  grooves.  They  have  special  forms  of  holders. 
They  rotate  on  their  centre,  or  they  may  be  swung  aside 
on  a  movable  arm  attached  to  the  stage.  There  are  al- 
ready so  many  that  about  the  only  kind  remaining  for 
the  reader  to  invent  is  one  that  shall  be  wound  up  and 
go  by  clock-work  and  a  spring,  or  by  electricity,  or 
steam,  or  water-power  or  wind. 

It  has  been  said  that  the  working  microscopist  can 
get  along  with  very  few  of  the  apparently  desirable  de- 
vices offered  by  the  dealers.  That  is  true.  Few  things 
microscopical  are  absolutely  indispensible,  and  among 
these  I  should  not  class,  for  general  use,  these  refine- 


MICROSCOPICAL    PRAXIS.  47 

ments  in  the  way  of  diaphragms.  If  the  beginning  mi- 
croscopist  is  not  careful  he  will  be  lost  in  a  wilderness 
of  diaphragms.  Yet  the  device  in  its  simplest  form  is 
essential  to  the  proper  action  of  the  instrument,  and  to 
the  proper  interpretation  of  the  magnified  image  of  the 
object.  Most  young  microscopists  use  too  much  light. 
They  seem  to  think  that  if  a  little  is  good,  much  is  bet- 
ter. The  result  is  that  the  eye  is  exposed  to  risk,  and 
the  finer  details  of  the  object  are  drowned  in  a  flood  of 
glaring  brightness.  The  field  should  be  toned  down  to 
a  soft  and  pleasant  glow,  and  to  accomplish  this,  aside 
from  the  use  of  the  blue  glass,  the  circular  disk-dia- 
phragm in  the  stage,  or  attached  to  it,  is  all  that  is 
needed.  This  should  be  turned  until  the  opening  which 
gives  the  proper  effect  is  beneath  the  object,  and  no 
aperture  should  be  used  because  it  seems  to  be  the 
right  one.  Try  them  in  succession,  and  adopc  the  one 
proper  to  the  occasion.  It  is  said  that  the  opening  to 
be  used  should  be  the  one  which  corresponds  in  size  to 
that  of  the  front  lens  of  the  objective.  This  is  true  if 
the  diaphragm  is  a  permanent  fixture  and  on  a  level 
with  the  upper  surface  of  the  stage,  but  if  it  can  be 
racked  upward  and  downward,  the  light  from  the  con- 
cave mirror  may  be  modified  satisfactorily  by  this 
movement.  Few  of  the  cheaper  stands  however,  have 
a  diaphragm  thus  movable,  and  those  adapted  to  the 
use  of  a  sub-stage  condenser  have  the  metal  disk,  or 
other  form,  so  arranged  that  it  may  be  entirely  re- 
moved, and  special  diaphragms  employed  below,  or 
sometimes  above,  the  lenses  of  the  condenser,  although 
the  last-mentioned  position  is  not  to  be  commended. 

Dealers,  and  microscopists  as  well,  differ  in  their 
opinion  as  to  the  proper  position  of  the  diaphragm  when 
it  is  to  be  attached  to  the  stage.  Some  place  it  half  an 


48  MICROSCOPICAL    PRAXIS. 

inch  or  more  below,  others  nearer  on  a  level  with  the 
upper  surface.  Its  action  differs  according  to  its  posi- 
tion and  the  character  of  the  light.  As  to  its  position 
the  student  must  usually  abide  by  the  decision  of  the 
maker,  unless  he  select  a  stand  with  the  diaphragm  on 
the  sub-stage,  or  one  adapted  to  the  use  of  a  sub-stage 
condenser.  If  it  is  attached  to  the  stage,  it  should  be 
as  close  to  the  object  as  possible. 

The  reader  will  of  course  understand  that  parallel 
rays  cannot  be  affected  in  any  way  by  the  position  of 
the  diaphragm  in  reference  to  the  object.  With  con- 
verging rays,  however,  the  result  is  different.  In  this 
case  the  lowering  of  the  diaphragm  will  obstruct  more 
of  the  oblique  rays  the  lower  it  is  depressed,  until  it 
comes  in  contact  with  the  mirror  itself.  A  similar 
effect  is  obtained  with  converging  rays  by  diminishing 
the  size  of  the  aperture  in  a  diaphragm  immediately 
beneath  the  stage,  the  smaller  the  opening  the  greater 
the  number  of  oblique  rays  obstructed.  With  parallel 
rays,  however,  the  depression  of  the  diaphragm  is  with- 
out noticeable  effect,  the  amount  of  light  being  dimin- 
ished by  the  use  of  decreased  apertures  in  the  dia- 
phragm-plate, so  that  a  diaphragm  immediately  below 
the  object  can  be  used,  by  changing  the  size  of  the  ap- 
ertures, to  modify  either  parallel  or  converging  rays. 
With  the  sub-stage  condenser  diminished  illumination 
is  obtained  by  depressing  the  condenser,  the  focus  of 
the  appliance  thus  being  removed  below  and  from  the 
object.  As  only  parallel  rays  should  be  used  with  this 
sub-stage  accessory,  their  power  and  number  are  dimin- 
ished by  the  use  of  diaphragms  of  different  apertures, 
as  well  as  by  the  differing  positions  of  the  instrument 
below  the  object,  the  diaphragms  being  below  the  con- 
denser. The  proper  place  then  for  the  plate  of  dia- 


MICROSCOPICAL    PRAXIS.  49 

phragm-op^nings  is  immediately  beneath  the  object 
when  the  condenser  is  not  used;  when  the  condenser  is 
used  the  diaphragm  should  be  as  close  as  possible  to  its 
back  lens.  Some  stands  have  it  attached  to  a  cylindri- 
cal box  at  some  distance  below  the  stage.  Such  stands 
are  good  ones  to  be  rejected. 

The  proper  amount  of  light  to  be  employed  in  ordi- 
dary  investigations  cannot  be  taught  in  words.  The 
beginner  must  learn  by  experience,  using  his  own  best 
judgment.  Avoid  a  glare,  and  avoid  semi-darkness. 
There  is  a  commendable  medium,  which  the  beginner 
must  find  for  himself,  if  he  work  alone. 


The  Eye-Piece,  or  Ocular. 

This  part  of  the  microscope  fits  loosely  into  the 
upper  end  of  the  body-tube,  from  which  it  may  be 
readily  removed.  Its  function  is  to  magnify  the  image 
produced  by  the  objective.  In  form  it  is  a  short  tube 
bearing  a  lens  or  a  combination  of  lenses  at  each  end 
and  varying  in  construction  and  in  name  almost  as 
greatly  as  does  the  diaphragm.  The  negative  eye- 
piece is  the  one  commonly  used.  It  is  so  named  be- 
cause its  focus  is  within  the  tube,  at  a  level  with  its 
diaphragm.  It  cannot  be  used  as  a  magnifying  glass 
without  the  objective. 


50  MICROSCOPICAL    PRAXIS. 

Eye-pieces  should  properly  fit  the  booty-tube.  A 
tightly  fitting  ocular,  and  one  that  is  too  loose,  are 
equally  annoying  and  inconvenient.  It  should  not  be 
loose  enough  to  shake  about  in  the  tube,  although  this 
may  be  easily  remedied  by  winding  with  paper;  it 
should  be  of  a  size  sufficient  to  allow  it  to  drop  in  by 
its  own  weight,  and  out,  too,  if  the  microscope  is  turned 
upside  down. 

The  negative  eye-piece  is  often  called  the  Huyghen- 
ian  because  the  celebrated  Dutch  astronomer  Huy- 
ghens,  who  died  in  1695,  first  used  one  of  similar  con- 
struction on  his  telescope.  Its  focus,  or  the  point  at 
which  the  image  is  formed,  falls  about  midway  between 
its  two  lenses,  while  in  the  positive  the  focus  is  below 
the  lower  glass.  In  the  negative  eye-pieces  the  lenses 
are  plano-convex  and  have  their  plane  surfaces  di- 
rected upward,  while  a  diaphragm  with  a  single  central 
aperture  is  placed  within  the  tube  at  a  point  corres- 
ponding to  the  place  where  the  image  is  formed.  This 
diaphragm  serves  to  intercept  all  those  marginal  rays 
not  active  in  the  production  of  the  image,  and  partly  to 
correct  the  spherical  aberration  as  well  as  the  chro- 
matic, the  diaphragm  being  further  utilized  as  a  place 
of  support  for  the  eye-piece  micrometer  used  in  the 
measurement  of  microscopic  objects. 

The  diaphragm  is  an  important  factor  everywhere  in 
the  microscope,  but  here  it  is  especially  useful,  not  only 
because  it  acts  as  stated,  but  also  because  upon  the  size 
of  the  aperture  in  its  centre  depends  the  size  of  the 
field.  By  contracting  that  the  field  may  be  diminished 
in  extent.  The  diameter  of  the  body-tube  is  not  alto- 
gether responsible  for  the  size  of  the  field,  and  neither 
is  the  diameter  of  the  eye-piece  itself.  The  diaphragm 
is  here  the  important  factor,  the  same  sized  opening 


MICROSCOPICAL    PRAXIS.  51 

being   used    in    all    negative    eye-pieces    of   the    same 
power. 

The  size  of  the  field  appears  larger  to  most  persons 
than  it  really  is,  and  this  apparent  size  is  misleading  to 
many,  whose  estimates  are  often  amusing  and  amazing. 
There  is  a  vast  difference  between  the  apparent  and 
the  actual  fields.  The  apparent  is  due  to  the  eye-piece, 
while  the  actual  is  represented  by  the  actual  amount  of 
surface  shown  at  one  time  by  the  objective,  this  area 
varying  with  every  magnifying  power  and  every  objec- 
tive. The  actual  field  may  be  measured  by  placing  a 
stage  micrometer  below  the  objective  and  counting  the 
number  of  spaces  included  within  the  circular  lighted 
area,  but  to  obtain  the  apparent  diameter  as  produced 
by  the  eye-piece,  a  simple  calculation  is  necessary. 

The  rule  is  to  multiply  the  diameter  of  the  dia- 
phragm-opening by  the  magnifying  power  of  the  eye- 
lens.  If  the  aperture  is  half  an  inch  across,  and  the 
eye-lens  magnifies  ten  times,  then  the  field  is  apparently 
five  inches  in  diameter.  To  me  these  matters,  even  so 
simple  a  one  as  this,  are  more  easily  manageable  if  put 
in  the  shape  of  an  algebraic  formula.  In  this  instance, 
then,  if  D  represent  the  diameter  of  the  diaphragm- 
opening,  P  the  magnifying  power  of  the  eye-lens,  and 
F  the  diameter  of  the  apparent  field,  we  will  have 
F=.DP,  or,  in  the  supposed  case,  ^=-|-Xio,  or  five 
inches. 

But  the  power  of  the  eye-lens,  how  can  that  be  ob- 
tained ?  This  is  one  of  those  interesting  little  points 
about  which  the  inquiring  student  soon  wants  to  know 
something.  The  magnifying  power  of  the  eye-lens  is 
equal  to  ten  inches  (the  arbitrary  standard  of  distance 
for  distinct  vision),  divided  by  its  focal  length  in  inches, 
so  that  it  only  remains  to  obtain  that  focal  length,  and 


52  MICROSCOPICAL    PRAXIS. 

both  problems  are  solved.  The  formula  here  would  be 
P=j,t  the  focal  distance  of  the  eye-lens  being  ob- 
tained in  the  manner  described  for  ascertaining  that  of 
the  bull's-eye  condenser  or  of  the  pocket-lens,  except 
that  here  the  convex  surface  must  always  be  turned 
toward  the  light,  since  that  is  its^  position  in  the  ocular. 
In  one  of  my  own  ocular's  measured  while  writing  this, 
the  focal  distance  of  the  eye-lens  is  ^  inches  which,  by 
the  formula  P~%  gives  P=io-t-\fa  or  yf  The 
diameter  of  the  diaphragm-opemng  is  by  actual  meas- 
urement 0.7  inch.  By  the  first-mentioned  formula,  J7— 
DP,  we  have  ^=7|x.7,  or  five  inches,  the  apparent 
diameter  of  the  field,  which  is  correct,  as  I  have  proved 
by  another,  simpler,  and  on  that  account,  probably  a 
better  way. 

This  is  to  make  the  measurement  by  means  of  the 
camera  lucida,  the  latter  being  applied  to  the  eye-piece 
and  the  microscope  placed  in  a  horizontal  position  with 
the  camera  lucida  ten  inches  from  the  table,  if  the 
Wollaston  form  of  camera  be  used,  the  limits  of  the 
field  being  marked  by  pencil  and  then  measured  by  the 
ordinary  foot-rule. 

If  the  reader  make  the  experiment  by  measuring  the 
focal  length  of  the  eye-lens,  he  should  take  special  care 
to  focus  the  lens  as  accurately  as  possible,  since  the 
difference  of  a  small  fraction  of  an  inch  makes  an 
astonishing  difference  in  the  result. 

The  size  of  the  actual  field  varies  with  the  nominal 
focus  and  with  the  power  of  the  objective.  The  field 
of  Zeiss's  variable  A*  when  at  its  lowest  power  is,  by 
actual  measurement,  thirty  twenty-fifths  of  an  inch, 
(1.2  in.),  and,  according  to  the  approximate  estimates  of 
Mr.  Edward  Bausch,  a  power  of  twenty-five  diameters 
will  show  a  surface  about  one-fifth  inch  wide;  fifty  di- 


MICROSCOPICAL    PRAXIS.  53 

ameters  will  include  an  area  one-tenth  of  an  inch  in 
width;  one  hundred  diameters,  one-twentieth  inch;  five 
hundred  diameters,  one  one-hundredth,  while  one 
thousand  diameters  will  exhibits  space  only  one  two-, 
hundredths  of  an  inch  wide. 

At  present  eye-pieces  of  different  powers  are  named 
after  the  letters  of  the  alphabet,  the  lowest  magnifying 
power  being  A,  the  next  highest  B,  and  so  on  down 
the  alphabet,  the  power  increasing  as  the  letters  ap- 
proach Z  and  ampersand.  This  method  might  not  be 
objectionable  if  it  conveyed  a  correct  idea  of  the  power, 
as  it  does  not,  some  makers  producing^  eye-pieces  that 
magnify  as  the  B  of  other  opticians,  no  two  dealers  hav- 
ing precisely  the  same  standard.  As  a  rule,  however, 
the  A  ocular  roughly  approaches  two  inches  in  focal 
length,  with  a  power  of  five,  and  so  continuing  through 
D,  £,  F  and  others  with  a  focal  length  of  f ,  £,  -^  \,  £ 
inches,  with  powers  varying  from  fifteen,  twenty, 
twenty-five  to  thirty,  forty  or  more  diameters. 

A  better  plan  would  be  to  mark  each    with    its    focal 
length,   two  inches,  one  inch,   or  whatever  it    may    be, 
and  to   have  it   understood    that    a   certain    length  al- 
ways represents   a  certain  invariable  power.      Thus  the  »  * 
one  inch  should  possess  a    power  of  ten  diameters;  the    j 
two  inch  of  five;  the  one-half  by  twenty;    the    one-fifth/ 
by  fifty.      And    if  not  only  the  focal  length  could  be 
stamped  on  the  cap  of  the   eye    lens,    but  the  power  as 
well,  the  change   would   be   acceptable    and  the  advan- 
tages   great.     Eye-pieces   are  made    varying   in    focal 
length  from  two    inches    to    one-sixteenth    inch.     The 
power  of  the  last  should  be  one  hundred  and    sixty   di- 
ameters.    With  it  is  I  do  not  know,  and  do  not  care  to 
know,  if  that  knowledge  must  be  contingent  upon    per- 
sonal ownership,  for  I  would  not  accept  such  an   ocular 


54 


MICROSCOPICAL    PRAXIS. 


as  a  gift,  if  I  were  to  be  forced  to  use  it.  A  more 
worthless  cumberer  of  the  apparatus  box  I  cannot  im- 
agine. No  human  eye  could  see  anything  satisfactorily 
with  such  an  eye-piece.. 

The  reader  may  desire  to  know  how  to  measure  the 
power  of  his  oculars  and  thus  learn  what  the  A,  B   and 
C,  or  the  2^/2  or  i  inch  represent  in  amplification.     For 
this  purpose  the  late  W.  H.  Bulloch, 
of  Chicago,  has  devised  a  simple  ap- 
paratus usable  by  anyone,    and    has 
described  an  easy  method  whose  re- 
sults are  at  least  approximately  cor- 
rect.    The   apparatus   is   shown    in 
figure  3.     It  consists  of   the   micro- 
scope B  supplied  with  a   two-inch 
objective,  and  placed  in  a  horizontal 
position  opposite  to  the  tube    G   D 
which  carries  at  E  F  the   eye-piece 
to  be  measured,  while  at  the  end  D  is 
a  diaphragm  pierced    by   an  oblong 
opening  of  known  size,    as  at  H.     C 
is  a  stationary  stage    bearing   a  mi- 
crometer ruled  to  one  hundredths  of 
an    inch    or     to    tenths   of    a  milli- 
metre.    In  Mr.  Bulloch's  instrument 
the    parts   are    all   brass  and    fitted 
with  rack    and   pinion    adjustments, 
which  are  very  well  in  their  way,  but 
not  essential  to    the    working   microscopist,    who  can 
make  the  tube  G  D  of  stiff  paper,  and  fasten  it   to   the 
top  of  a  pile  of  books  as  the  writer  has  done,  using   the 
microscope  for  the  other   part   of   the   instrument,    the 
diaphragm  at  D  being  also  of  paper,  while  the  stage    C 
is  that  of  the  microscope.     The  distance  from   the  di- 


MICROSCOPICAL    PRAXIS.  55 

aphragm  D  to  the  diaphragm  within  the  eye-piece  to  be 
measured  should  be  ten  inches,  the  arbitrary  standard 
of  length  for  distinct  vision. 

Screw  to  the  microscope  the  two-inch  object- 
ive, as  it  is  the  most  convenient  for  this  purpose,  and 
focus  it  on  the  lines  of  the  micrometer  at  C.  Then 
bring  the  tube  G  D  toward  the  micrometer  until  the 
image  of  the  aperture  at  H  is  focussed  on  the  same 
lines,  presenting  the  appearance  of  H'  in  the  figure. 
Count  the  number  of  spaces  occupied  by  the  image, 
and  divide  the  number  into  the  known  size  of  the  aper- 
ture in  the  diaphragm  at  H.  In  Mr.  Bulloch's  case  the 
diaphragm-opening  was  6.5  millimetres,  which  covered 
eleven  spaces  of  a  micrometer  ruled  to  tenths  of  a  mil- 
limetre. A  simple  formula  and  its  application  is,  P  the 
power  to  be  obtained;  D  the  size  of  the  diaphragm 
opening,  and  m  the  number  of  micrometer  spaces  oc- 
cupied; then  P=.~ .  In  this  case  the  value  of  D  is 
6.5  mm.;  of  m  eleven-tenths  mm,  and  the  calculation, 
in  the  form  of  a  decimal  fraction,  is  -f-:f  =  -f-f-  or  about  six. 
The  reader  of  course  understands  that  when  his  micro- 
meter is  ruled  in  parts  of  an  inch,  the  diaphragm  open- 
ing atZ>  in  the  figure,  may  be  of  any  convenient  size 
provided  it  also  is  in  parts  of  an  inch.  In  the  case  of  one 
of  Mr.  Bulloch's  eye-pieces  belonging  to  my  own  stand, 
and  measured  very  hastily  while  waiting  for  the 
dinner-bell  to  ring,  being  done,  therefore,  under  very 
adverse  circumstances,  the  result  was  4.4+,  the 
power  marked  on  the  cap  of  the  eye  lens  being  4.5. 
With  greater  care  in  reading  the  micrometer  the  re- 
sult might  have  been  actually  that  of  Mr.  Bulloch 
although  the  difference  is  very  slight.  The  tube  G  D 
was  a  pasteboard  cylinder  into  which  the  eye-piece 
to  be  measured  was  wedged  with  a  corner  of  the 


56  MICROSCOPICAL    PRAXIS. 

pocket  handkerchief.  The  micrometer  was  ruled  in 
hundredths  of  an  inch,  and  the  diaphragm  aperture  was 
two-tenths  of  an  inch  long  (Z>),  occupying  four  and 
one  half  micrometer  spaces  (;;/).  The  calculation  was, 


=     or          or 


However  careful  of  his  stand  the  microscopist  may  be, 
the  dust  will  collect  on  it  and  especially  on  the  eye  lens. 
This  when  abundant  is  indistinctly  visible  and  should  be 
removed.  There  is  one  right  way  to  do  this,  and  that  is 
not  by  a  careless  wipe  with  the  pocket  handkerchief, 
as  I  have  seen  done.  A  particle  of  grit  may  be  on  the 
glass  or  in  the  cloth,  when  a  scratch  will  be  made  that 
may  be  disastrous  in  its  effects.  To  avoid  such  dan- 
ger, blow  away  the  dust  with  several  short  quick  puffs 
of  the  breath,  then  carefully  wipe  the  surface  with  the 
Japanese  filter  paper,  as  originally  recommended  by 
Prof.  S.  H.  Gage,  of  Cornell  University.  This  paper 
is  extensively  used  by  the  dentists,  and  may  be  had 
cheaply  from  dealers  in  dental  supplies.  It  is  soft  and 
thin,  with  no  tendency  to  collect  the  dust,  and  with  no 
kaolin  or  gritty  matter  in  its  substance.  It  may  there- 
fore be  used  almost  recklessly.  Prof.  Gage  has  also 
recommended  it  for  cleaning  the  fronts  of  objectives, 
for  which  purpose  it  can  not  be  excelled.  A  sheet  of 
it  should  be  cut  into  small  squares  and  kept  in  a 
covered  box  on  the  microscopist's  table,  a  piece  being 
used  once  only,  to  be  thrown  away  after  each 
application  to  the  objective  or  ocular.  ^ 

It  is  sometimes  difficult  to  describe  a  special  object 
in  the  field,  or  some  special  part  of  an  object  so  that 
another  person  may  recognize  it.  This  is  particularly 
difficult  if  the  other  person  has  had  no  experience  in 
the  use  of  the  microscope.  He  will  be  as  likely  to 
gaze  with  awe  at  an  air  bubble  or  a  bit  of  colored 


MICROSCOPICAL    PRAXIS.  57 

• 

wool,  as  at  the  specified  object.  To  obviate  this,  a 
pointer  called  an  indicator  is  occasionally  used.  As 
originally  devised  by  Quekett  it  consists  of  a  fine  steel 
hand  so  pivoted  within  the  eye-piece,  and  near  the  dia- 
phragm, as  to  be  in  the  focus  of  the  eye  lens,  and 
capable  of  being  swuhg  forward  to  the  centre  of  the 
field  so  as  to  point  to  the  special  object,  and  then 
swung  back  out  of  sight  when  not  in  service.  It  is  not 
much  used  now,  but  an  anonymous  writer  describes  a 
simple  form  made  by  attaching  a  hair  to  the  diaphragm 
so  as  to  extend  half  way  across  the  field.  He  gums 
the  piece  of  hair  to  a  bit  of  paper,  the  end  projecting 
to  the  proper  distance  beyond  the  edge.  When  dry 
he  cuts  the  paper  to  the  right  size,  moistens  the  gum- 
med side,  and  attaches  the  whole  to  the  diaphragm. 
He  says  that  "This  simple  arrangement  is  entirely  satis- 
factory, and  we  have  it  now  in  constant  use.  The  hair 
is  not  objectionable  in  the  ocular,  as  it  appears  as  a 
fine  sharp  line,  and  is  quite  overlooked  when  one  gets 
accustomed  to  its  presence."  To  me  personally  the 
presence  of  the  fine  line  would  be  an  annoyance.  How- 
ever, if  the  microscope  is  much  used  for  showing  ob- 
jects to  others,  so  simple  an  arrangement  might  be 
commendable. 


58  MICROSCOPICAL    PRAXIS. 

The  Micrometer. 

The  micrometer  is  an  instrument  for  the  measuring 
of  minute  objects  or  of  short  spaces.  In  some  form  it 
is  in  frequent  use  by  the  microscopist.  It  consists  of  a 
series  of  fine  lines  ruled  on  glass,  the  distance  between 
the  lines  varying  according  to  the  wish  of  the  microsco- 
pist, or  of  the  optician  that  makes  the  plate.  These 
bands  of  rulings  are  prepared  by  special  and  compli- 
cated machines,  some  of  which  are  so  delicate  and  so 
easily  influenced  for  the  worst,  that  they  are  allowed'  to 
work  only  in  the  darkness  and  the  stillness  of  the  night. 
Nobert,  of  Prussia,  was  for  many  years  the  most  noted 
maker  of  fine  rulings,  some  of  his  work  having  fora 
long  time  served  as  tests  for  the  best  and  highest-power 
objectives.  In  this  country  we  have  had  three  micros- 
copists  who  have  turned  their  attention  to  the  making 
of  fine  micrometers  and  bands  of  closely  ruled  lines, 
Prof.  W.  A.  Rogers,  who  did  splendid  work  but  who  has 
now  retired  from  this  field  arid  transferred  his  ruling- 
machine  to  Prof.  M.  D.  Ewell  of  Chicago,  the  other 
worker  of  note  in  this  department  being  the  late  Charles 
Fasoldt  of  Albany,  N.  Y.,  whose  extravagant  claims  in 
regard  to  his  rulings  detract  from  the  estimation  in 
which  he  otherwise  would  have  been  held  by  intelligent 
microscopists.  His  assertion  that  he  had  ruled  a  band 
containing  one  million  lines  to  the  inch  is  known  by 
every  well-informed  microscopist  to  be  an  absurdity  and 
impossibility.  No  human  eye  has  seen  more  than  120,- 
ooo  ruled  lines  to  the  inch,  and  no  human  eye  probably 
ever  will.  Professor  M.  D.  Ewell  is  at  present  the  only 
person  in  this  country  ruling  fine  micrometers,  and  all 
his  work  is  commendable,  while  his  prices  are  exceed- 
ingly reasonable. 

The  microscopist  should  possess  two  forms  of  micro- 


MICROSCOPICAL    PRAXIS.  59 

meter,  one  for  the  stage,  the  other  for  use  in  the  eye- 
piece, where  it  is  always  employed  without  the  stage- 
micrometer,  after  the  value  of  its  spaces  have  been  as- 
certained. This  is  a  great  convenience  over  the  old 
and  awkward,  and  often  impossible,  method  of  using 
the  stage-micrometer  by  placing  the  object  above  it  and 
focussing  down  through  the  thickness  of  both  to  see 
how  the  lines  stood  in  relation  to  the  margins  of  the 
specimen.  This  was  oftener  than  not  impossible  for 
use  with  high-power  objectives,  which  always  have  a 
short  working-distance. 

The  two  kinds  of  micrometers  differ  in  form  as  well 
as  in  position.  That  for  the  stage  is  mounted  on  a  slip 
of  glass  of  the  standard  size,  while  that  for  the  ocular 
is  a  circular  disk  of  thin  glass  made  to  rest  on  the  dia- 
phragm of  the  eye-piece  tube. 


The  Stage-Micrometer. 

The  stage-micrometer  is  sometimes  ruled  directly  on 
the  glass  slip  and  used  without  a  cover-glass;  some- 
times it  is  ruled  on  the  cover  itself  which  is  then 
mounted  on  the  slip,  the  first-mentioned  kind  probably 
being  rather  more  easily  used.  The  lines  in  either 
case  are*  usually  filled  with  graphite  to  make  them  more 
distinct,  for  it  has  happened  that  although  the  micros- 


60  MICROSCOPICAL    PRAXIS. 

copist  has  known  that  the  lines  were  there,  they  have 
been  so  faintly  ruled  that  they  were  invisible  even 
under  the  best  objectives  manipulated  by  an  expert 
microscopist. 

Usually  the  lines  will  be  in  two  narrow  bands  ruled 
at  the  rate  of  one  hundred  and  one  thousand  to  the 
inch.  These  are  sufficient  for  most  purposes;  yet  it 
will  be  convenient  for  use  with  high-powers  to  have  a 
third  band  ruled  at  the  rate  of  two  thousand  to  the 
inch,  and  this  will  not  add  much  to  the  cost.  If  the 
reader  should  prefer  to  have  all  his  measurements 
made  in  millimetres  there  will  be  no  objection  to  it,  but 
in  that  case  the  micrometer  may  well  be  ruled  to  tenths 
and  hundredths  of  a  millimetre.  When  ordering  the 
stage-micrometer  therefore,  it  is  well  to  give  instruc- 
tions on  •.  hese  points,  otherwise  the  maker  will  use  his 
own  judgment.  For  practical  work  nothing  will  be 
needed  less  than  the  one  two-thousandth  of  an  inch  or 
the  hundredth  of  a  millimetre. 

The  stage-micrometer  is  rarely  used  for  the  direct 
measurement  of  the  object  in  the  way  already  referred 
to;  the  reasons  for  this  have  also  been  given.  But 
when  this  can  be  done  with  very  low-power  objectives, 
the  object  is  laid  across  the  micrometer  lines,  and 
measured  much  as  a  two-foot  rule  would  measure  it. 
Thus,  if  the  object  extends  across  just  two  and  one-half 
of  the  one-hundredth  inch  spaces,  it  of  course  measures 
two  and  one-half  hundredths,  or  -J^.of  an  inch. 

When  the  object  extends  into  a  space  without  en- 
tirely crossing  it  to  the  next  line,  the  portion  of  the 
space  occupied  must  be  estimated  by  the  eye,  and  here 
enters  a  source  of  error  against  which  the  microscopist 
must  guard.  The  same  holds  true  in  the  use  of  the 
eye-piece  micrometer  when  the  object  extends  beyond 


MICROSCOPICAL    PRAXIS.  6l 

a  line  but  not  entirely  across  a  space.  The  more  ex- 
pert the  microscopist  is  in  estimating  distances,  the 
more  nearly  free  from  error  will  his  measurements  be 
in  such  instances.  At  other  times,  if  the  micrometer  be 
correctly  ruled,  there  will  be  little  trouble  in  reading 
the  spaces,  especially  since  every  fifth  line  is,  or  should 
be,  longer  than  the  remaining  four,  its  end  extending 
above  the  general  level  of  the  others.  This  catches 
the  eye  and  enables  the  microscopist  to  read  the  lines 
rapidly  in  groups  of  five.  This  direct  method  of  meas- 
uring, however,  has  been  almost  entirely  superseded, 
that  by  means  of  the  camera  lucida  being  more  accur- 
ate and  better  adapted  to  high-powers,  if  the  microsco- 
pist have  no  eye-piece  micrometer. 

By  the  camera  lucida  process  just  mentioned  the 
micrometer  lines  are  focussed,  after  the  microscope  has 
been  placed  in  a  horizontal  position,  with  the  eye-piece 
about  ten  inches  from  the  table-top.  The  camera 
lucida  is  then  attached  and  the  lines  drawn  on  the 
paper  upon  the  table.  The  micrometer  is  now  to  be 
removed,  the  object  substituted,  and  the  number  of 
spaces  its  image  occupies  on  the  paper  scale  at  once 
read.  Thus,  if  the  one-thousandth  inch  spaces  have 
been  sketched,  and  the  image  of  the  object,  drawn  with 
the  microscope  in  the  same  position  and  with  the  same 
objective,  occupies  eight  of  those  spaces,  it  of  course 
measures  eight  one-thousandths  of  an  inch,  or  T-|T.  A 
scale  should  be  made  for  every  objective  and  eye-piece, 
so  that  the  image  may  be  projected  on  the  proper  one 
by  the  camera  lucida,  without  the  trouble  of  drawing 
the  micrometer  lines  every  time  this  method  is  used. 
But  at  best  this  is  an  awkwrard  way  of  accomplishing 
what  may  be  done  with  great  ease  and  rapidity  with  the 
eye-piece  micrometer. 


62  MICROSCOPICAL    PRAXIS. 

Efforts  have  been  made  to  -induce  microscopists  to 
adopt  some  standard  to  which  micrometers  shall  be 
ruled,  so  that  all  measurements  may  be  expressed  in 
the  same  terms.  In  this  country  and  in  England,  parts 
of  an  inch  are  used,  unless  the  microscopist  orders 
otherwise;  on  the  continent  of  Europe  parts  of  a  milli- 
metre. The  American  Society  of  Microscopists  and 
others  have  endeavored  to  induce  microscopists  to 
adopt  the  one  one-thousandth  of  a  millimetre,  or  about 
one-twenty-five  thousandth  of  an  inch  as  the  standard 
for  measurements,  but  thus  far  without  much  success, 
although  it  will  probably  become  common  in  time. 
This  one  one-thousandth  of  a  millimetre  is  to  be  known 
as  the  micron,  and  to  be  represented  by  the  Greek  let- 
ter //.  In  Europe  it  is  frequently  used,  but  in  this 
country  writers  in  the 'scientific  periodicals  have  not 
employed  it  generally  enough  to  make  the  majority  of 
readers  of  microscopical  literature  familiar  with  it.  As 
a  unit  of  distance,  the  measure  is  in  every  way  com- 
mendable, and  should  be  used  whenever  possible. 


The  Eye-piece  Micrometer. 

This,  as  its  name  indicates,  is  used  only  in  the  eye- 
piece, where  it  is  placed  on  the  diaphragm.  It  is  made 
to  fit  in  the  tube,  or  in  an  adapter  which  then  rests  on 
the  diaphragm.  It  is  used  by  rotating  the  eye-piece 


MICROSCOPICAL    PRAXIS.  63 

until  the  lines  lie  across  the  object  to  be  measured  and 
at  right  angles  to  its  length  or  breadth.  The  spaces 
occupied  are  counted,  and  a  simple  calculation  gives 
the  length  or  the  breadth,  when  the  value  of  each  space 
is  known. 

To  ascertain  this  value,  a  stage  micrometer  is  needed, 
and  when  the  latter  has  been  used  for  this  purpose  it  is 
of  no  further  importance  unless  the  owner  should  de- 
sire to  ascertain  the  value  of  the  spaces  in  another  eye- 
piece micrometer.  The  stage-plate  may  then  be  put 
away,  as  the  eye-piece  disk  is  now  all  that  will  be 
needed  for  all  microscopical  measurements. 

Put  the  stage-plate  in  position  and  focus  the  lines. 
Remove  this  eye-piece,  insert  the  one  that  carries  the 
eye-piece  micrometer,  and  again  focus.  The  stage- 
micrometer  is  then  to  be  moved  until  one  of  its  lines 
exactly  coincides  with  one  of  the  lines  in  the  eye-piece 
micrometer,  and  as  nearly  as  possible  in  the  centre  of 
the  field.  Notice  how  many  spaces  in  the  eye-piece  are 
needed  to  fill  one  space  of  the  stage-plate,  and  obtain 
the  value  of  one  of  the  intervals  in  the  former,  by  di- 
viding the  value  of  the  single  space  on  the  stage-micro- 
meter by  the  number  of  spaces  it  fills  in  the  eye-piece 
micrometer.  Thus,  if  one  of  the  one-hundredth  inch 
intervals  on  the  stage-plate  is  just  filled  by  three 
spaces  on  the  eye-piece  micrometer,  the  value  of  one 
space  of  the  latter  will  be  -^J-g-  of  an  inch,  or  the  yj-^  of 
the  stage-plate  divided  by  the  three  divisions  occupied 
in  the  eye-piece  micrometer.  Or  if  ten  spaces  in  the 
eye-piece  fill  one  of  the  10*00  inch  divisions  on  the 
stage-micrometer,  then  the  value  of  the  one  eye-piece 
space  is  io^00  inch,  or  -j-gVo  divided  by  ten,  and  any  ob- 
ject that  just  fills  one  such  space  in  the  eye-piece 
would  be  To^00  inch  in  length;  if  it  occupied  five  spaces 


64  MICROSCOPICAL    PRAXIS. 


it  would  be  T^OTT  or  TsVo  inch  lon§-  The  value  of  the 
spaces  must  be  ascertained  in  this  way  with  every  ob- 
jective, and  the  measurement  of  the  object  should  al- 
ways be  made  under  the  same  conditions  as  far  as  re- 
gards tube-length  and  the  position  of  the  adjustment- 
collar  of  the  objective,  any  change  in  either  at  once  al- 
tering the  value  of  the  micrometer-spaces. 

It  sometimes  happens  that  a  certain  number  of  di- 
visions in  the  eye-piece  cannot  be  made  to  fill  exactly  a 
certain  number  on  the  stage-micrometer,  a  fraction  of  a 
space  being  left  over.  For  instance,  thirteen  divisions  in 
the  eye-piece  may  perhaps  occupy  two  and  one-half  of  the 
one  one-hundredth  inch  spaces  on  the  stage-plate.  To 
simplify  the  calculation  in  such  a  case,  the  draw-tube 
may  be  extended  until  a  certain  number  of  eye-piece 
spaces  will  exactly  fill  a  certain  number  on  the  stage- 
micrometer,  when  the  position  of  the  tube  must  be  re- 
corded, and  all  subsequent  measurements  made  with  it 
at  that  place.  Or,  if  the  microscopist  have  an  eye  sen- 
sitive to  small  distances,  he  may  estimate  the  part  of  a 
space  occupied. 

The  position  of  the  adjustment-collar  of  the  objective 
also  alters  the  value  of  the  spaces,  since  any  movement 
towards  closed  point  or  bringing  the  component  lenses 
closer  together,  increases  the  magnifying  power,  and 
decreases  it  when  made  toward  open  point.  With  ad- 
justable objectives,  therefore,  the  value  must  be  ascer- 
tained for  every  position  of  the  collar,  unless  the  mi- 
croscopist is  willing  to  place  the  adjustment-ring  at 
open  or  at  closed,  and  to  allow  it  always  to  remain 
there  when  measurements  are  to  be  made. 

The  eye-piece  also  affects  the  value  of  the  spaces, 
since  they  are  magnified  by  the  eye-lens  whose  power, 
•of  course,  varies  in  oculars  of  different  focal  lengths 


MICROSCOPICAL    PRAXIS.  65 

It  is  best,  indeed,  always  to  employ  the  same  ocular, 
allowing  the  micrometer  to  remain  on  the  diaphragm, 
using  that  eye-piece  for  measuring-work  only.  The 
one-inch,  or  the  one  and  one-half  inch  ocular  is  a  de- 
sirable one  for  the  purpose. 

The  value  of  the  spaces  with  each  objective  should 
be  recorded  for  ready  reference.  The  reader  will  prob- 
ably prefer  to  devise  his  own  method  of  doing  this,  but 
the  following  is  the  writer's  tabular  form,  given  only  to 
show  the  manner  of  preparing  the  record  in  this  partic- 
ular instance. 


Value  of  Micrometer  Divisions  with  each  Ob- 
jective. 


Objective. 

Adjustment. 

Inch 
(fractions.) 

Inch 
(decimals.) 

Micron  /* 

D-tube. 

I-IO 

Closed. 

i  -6000 

0.00016664- 

4 

In. 

The  number  of  intervals  on  the  eye-piece  micrometer 
is  of  no  importance,  provided  they  are  equally  spaced. 
The  ruling  may  therefore  be  safely  left  to  the  maker. 
The  late  Dr.  J.  J.  Woodward,  however,  thought  other- 
wise, for  he  has  said  that,  "So  long  as  the  English 
microscopists  continue  to  express  the  results  of  their 
measurements  in  decimals  of  an  English  inch,  there 
will  be  American  microscopists  who  will  do  the  same, 


66  MICROSCOPICAL    PRAXIS. 

either  for  all  purposes  or  for  particular  work,  and  of 
course  it  is  very  desirable  that  the  measurements  also 
should  be  accurate.  The  stage-micrometers  on  this 
system  in  the  market  are  usually  ruled  in  hundredths 
and  thousandths  of  an  inch.  The  latter  divisions  are 
too  wide  to  give  values  to  the  eye-piece  micrometer  with 
the  higher  powers,  while  the  five-thousandths  and  ten- 
thousandths  and  even  finer  divisions  ruled  also  on  some 
of  these  micrometers,  are  inconveniently  close.  I  would 
advise  the  makers  to  rule  such  micrometers  four-tenths 
of  an  inch  long,  divided  into  hundredths  of  an  inch,  one 
of  the  hundredths  being  divided  into  ten,  another  into 
twenty-five.  These  latter  spaces,  each  representing 
one  twenty-five-hundredth  of  an  inch,  sufficiently  ap- 
proximate the  hundredth  of  a  millimetre  to  be  used 
with  equal  convenience  with  the  higher  powers.  The 
scale  on  the  glass  eye-piece  micrometer,  used  with  these 
stage-micrometers,  should  be,  if  specially  made  for  the 
purpose,  four-tenths  of  an  inch  long,  divided  into  one- 
hundred  parts,  each  one  two-hundred-and-fiftieth  of  an 
inch;  but  these  divisions  would  so  closely  approximate 
those  of  the  metric  eye-piece  micrometer  proposed, 
that  it  might  be  used  without  inconvenience  instead." 


MICROSCOPICAL    PRAXIS.  67 

To  Ascertain  the  Comparative   Enlargement  of 
a  Drawing. 

It  is  sometimes  desirable  to  learn  how  much  the 
drawing  exceeds  the  size  of  the  object,  especially  when 
drawings  are  to  be  made  to  a  certain  scale.  To  do  this, 
divide  the  length  of  the  drawing  by  the  actual  length  of 
the  object  as  ascertained  by  the  micrometer,  and  the 
quotient  will  be  the  enlargement  of  the  sketch.  Thus, 
if  the  drawing  is  five-tenths  of  an  inch  long,  and  the  ob- 
ject one  one-hundredth  of  an  inch,  the  sketch  is  en- 
larged fifty  diameters. 


How  to  Care  for  the  Instrument. 

When  the  microscope  is  received  from  the  dealer  it 
will  be  in  a  case,  with  a  lock  and  key  which  are  often 
mentioned  in  the  catalogues  as  if  they  were  rare  and 
unfamiliar  things.  After  the  box  has  been  lifted  to  the 
table  by  the  brass  handle  at  the  top,  the  door  is  opened 
and  the  owner  glances  within,  his  heart  beating  a  little, 
faster,  and  agreeable  anticipations  bringing  a  pleasing 
expression  to  his  face.  The  instrument  will  probably 
have  the  front  toward  the  back  of  the  case,  therefore 
turned  away  from  the  microscopist.  At  first  acquaint- 
ance it  will  turn  its  back  on  him  in  more  senses  than 
one. 


68  MICROSCOPICAL    PRAXIS. 

I  have  seen  men  clutch  the  upright  stand  by  the  body 
and  the  milled  heads  of  the  coarse  adjustment,  drag  out 
the  unresisting  thing  and  set  it  down  on  the  table  with 
a  bang.  Such  men  are  not  fit  to  possess  a  microscope. 
The  instrument  may  be  strong  and  well  made,  but  as 
some  one  has  said,  it  is  never  necessary  to  brutalize  it. 
If  it  be  supplied  with  a  base-board  as  it  should  be, 
gently  slide  it  out  of  the  case  by  pulling  board  and  all 
toward  you,  and  as  gently  place  it  on  the  table.  If  it 
be  not  attached  in  any  way  to  the  case,  carefully  lift  it 
out  by  means  of  the  arm.  If  you  treat  the  instrument 
kindly  it  will  repay  you  a  thousand  fold.  If  you  at- 
tempt to  coerce  it,  a  rebellion  will  be  speedy  and  your 
downfall  sure. 

Sometimes  one  eye-piece  will  be  found  in  the  body 
tube,  sometimes  in  a  side  box  or  drawer,  according  to 
the  size  and  style  of  the  case  and  the  stand.  In  any 
event  the  eye-piece  is  to  be  gently  dropped  into  the  top 
of  the  body-tube  as  the  stand  rests  vertically  on  the 
table.  The  microscopist  seats  himself  on  a  chair  and 
in  any  position  that  he  may  find  comfortable.  Every 
observer  will  form  habits  of  his  own  in  reference  to  his 
position  before  the  instrument,  and  will  have  his  own 
ideas  as  to  the  proper  size  and  style  of  his  work-table, 
and  perhaps  even  to  the  number  of  legs  that  the  table 
should  have.  Some  writers  have  advised  that  there 
shall  be  three  legs  to  the  microscopical  table  so  that  it 
may  be  steady  on  an  uneven  surface.  There  is  no  ob- 
jection to  three  legs,  if  the  microscopist  wants  them. 
He  may  also  sit  on  a  three-legged  stool,  if  he  should  de- 
sire to  do  so.  But  since  the  floors  of  modern  houses 
are  seldom  uneven  enough  to  disturb  the  equanimity  of 
a  quadrupedal  table,  that  seems  to  be  the  preferable 
form,  the  great  desiderata  being  firmness  and  solidity. 


MICROSCOPICAL    PRAXIS.  69 

I  remember  that  the  ladies  in  my  family  were  once  at- 
tacked by  the  aesthetic  notion  that  if  I  could  be  induced 
to  put  the  microscope  on  what  they  called  a  "Tea-poy" 
table,  and  under  a  glass  shade,  it  would  look  well.  The 
table  had  four  filamentous  legs,  and  a  shelf  half  way 
between  the  floor  and  the  top,  the  whole  being  a  silly 
invention  of  some  frivolous  mind.  The  thing  trembled 
at  a  touch,  the  shelf  scraped  my  shins,  the  microscope 
danced,  the  lamp  wabbled,  and  the  deluded  victim  ex- 
pressed his  opinion.  ^Esthetics  are  well  enough,  but 
they  should  be  looked  for  in  the  object  under  the  lens 
rather  than  in  the  table.  I  now  use  a  strong,  substan- 
tial, four-legged  pine  table  that  cost  less  than  three  dol- 
lars, and  I  would  not  change  it  for  a  Louis  XIV.  or  for 
a  Chippendale.  All  that  is  needed  is  that  it  shall  be 
solid  and  firm,  with  an  abundance  of  top  space,  and  a 
drawer  or  two  to  hold  the  many  "traps"  and  "dodges" 
that  soon  accumulate. 

Seated  before  the  stand,  incline  it  at  a  convenient  an- 
gle, the  stage,  the  mirror,  that  is,  the  front  of  the  mi- 
croscope, of  course  being  turned  from  you.  When  in- 
clining the  instrument,  do  so  if  possible  by  means  of  the 
arm;  at  any  rate  do  nothing  to  bring  a  strain  on  the 
coarse  or  on  the  fine  adjustment.  All  the  least  costly 
stands  will  remain  in  an  inclined  position,  held  there  by 
the  friction  within  the  joint  at  the  top  of  the  pillar;  in 
first-class  instruments  the  trunnions  carry  tightening  or 
binding  screws,  so  that  the  wear  that  sooner  or  later  be- 
comes noticeable  in  the  former  can  be  taken  up  in  the 
latter. 

With  the  eye-piece  in  place,  and  the  body  inclined, 
attach  the  objective.  To  do  this,  rack  up  the  body  un- 
til there  is  no  danger  that  the  front  of  the  lens  will 
come  in  contact  with  the  stage.  Unscrew  the  top  of 


70  MICROSCOPICAL    PRAXIS. 

the  brass  box  containing  the  objective  and  tip  the  latter 
out  into  the  palm  of  the  left-hand,  supporting  it  with 
the  ringers.  Take  it  up  with  the  right-hand,  and  turn- 
ing the  screw  end  upward,  screw  it  to  the  lower  end  of 
the  body-tube.  It  is  unnecessary  to  caution  the  reader 
in  regard  to  crossing  the  threads  of  the  screws.  If  that 
be  done  and  the  objective  wedged  into  the  nose-piece, 
the  owner  of  that  stand  may  have  a  sad  experience. 
Mr.  Wm.  Wales  relates  an  instance  of  this  kind  where 
the  objective  could  not  be  removed  by  hand,  so  the 
wise  owner  used  a  heavy  pair  of  gas-fitter's  pliers,  and 
succeeded  in  damaging  the  instrument  to  the  amount  of 
forty-five  dollars,  pulling  out  the  entire  fine-adjustment, 
which  in  this  case  was  on  the  lower  end  of  the  body. 
It  is  often  useful  to  rotate  the  objective  backward  for  a 
short  distance,  until  the  threads  are  felt  to  slip  into 
place,  when  the  lens  may  then  be  screwed  home  by  gen- 
tle forward  turns.  If  it  does  not  move  easily  and 
smoothly,  something  is  wrong,  and  no  force  should  be 
applied,  but  the  objective  must  be  removed  and  the 
difficulty  discovered  and  corrected. 

If  the  microscope  is  to  be  used  by  day-light  a  posi- 
tion near  a  north  window  is  the  best,  as  the  light  from 
the  northern  sky  is  the  most  uniform.  A  white  cloud 
illuminated  by  the  sun  is  the  most  desirable  light  by 
day,  but  it  can  seldom  be  obtained.  Most  microsco- 
pists  have  some  favorite  position  before  the  window, 
many  preferring  the  stand  so  arranged  that  the  mirror 
shall  face  the  window  and  the  sky;  others  place  it  so 
that  the  window  shall  be  at  the  side  of  the  instrument. 
This  is  entirely  a  personal  preference,  nothing,  so  far  as 
I  know,  being  gained  or  lost  in  either  position.  If, 
however,  daylight  is  used  with  a  sub-stage  condenser, 
the  plane  mirror  should  be  turned  toward  the  sky.  If 


MICROSCOPICAL    PRAXIS.  7  I 

no  sub-stage  condenser  is  employed,  then  the  concave 
surface  may  be  directed  upward. 

The  use  of  lamp-light  has  already  been  referred  to, 
and  the  employment  of  some  special  form  of  microscope- 
lamp  recommended.  Its  position  in  relation  to  the  mir- 
ror may,  if  desired,  be  ascertained  by  the  formulae  given 
in  a  preceding  chapter,  but  I  think  that  most  microscop- 
ists,  for  ordinary  work,  do  not  enter  into  the  niceties 
of  such  an  arrangement,  reserving  these  refinements  for 
some  very  special  and  delicate  investigations.  '  The 
reader  will  find  however,  that  any  attention  paid  to 
these  points,  even  for  every-day  observations,  will  re- 
sult in  good.  Usually  the  lamp  is  placed  somewhat  in 
advance  of  the  microscope,  and  always  on  the  left-hand 
side.  This  gives  plenty  of  room  on  the  table,  so  that 
the  lamp  is  in  no  danger  of  being  overturned. 

Do  not  handle  the  mirror  too  daintily;  if  well  mounted 
there  is  little  danger  of  injuring  it,  and  a  firm  grasp 
makes  it  more  easily  manageable.  At  first  there  will 
probably  be  some  difficulty  in  illuminating  the  field  as 
it  should  be  illuminated,  but  a  little  practice  will  ac- 
complish it.  The  entire  circular  space  called  the  field 
should  be  evenly  lighted;  there  should  be  no  shadows 
nor  faintness  in  the  glow  near  the  edge.  Some  writers 
recommend  that  a  piece  of  tissue  paper  should  be 
placed  over  the  stage-opening  and  .the  mirror  manip- 
ulated until  the  light  is  thrown  exactly  in  its  centre;  it 
is  then  removed  and  the  light  of  course  passes 
through  the  objective  and  up  the  body-tube.  This  is 
a  good  plan  if  the  reader  has  trouble  in  seeing  where 
the  reflection  is  thrown,  but  usually  the  light  may  be 
observed  on  the  front  of  the  objective.  The  field  can 
scarcely  be  illuminated  while  the  eye  is  at  the  eye-piece; 
the  illumination  may  then  be  completed,  but  not  begun- 


72  MICROSCOPICAL    PRAXIS. 

The  only  plan  is  to  use  the  paper  on  the  stage,  or  to 
observe  when  the  front  lens  receives  the  light.  Then 
apply  the  eye  and  gently  manipulate  the  mirror,  trying 
to  improve  matters;  but  even  now  the  best  can  not  be 
obtained.  This  must  wait  until  the  slide  is  on  the 
stage,  when  the  body  is  carefully  racked  down  and  the 
focus  obtained  as  previously  directed.  It  is  possible 
that  the  field  may  then  be  evenly  illuminated,  but  too 
faintly  to  show  the  object  properly,  or  oblique  shadows 
may  be  thrown  across  it,  or  only  one  little  space  at  the 
side  may  be  bright  while  all  the  rest  is  semi-obscure. 
This  must  be  remedied  by  gently  moving  the  mirror 
while  eye  is  at  the  ocular.  When  all  the  circular  region 
is  lighted  as  well  as  seems  possible,  remove  the  eye- 
piece and  centre  the  illuminating  beam  by  the  method 
suggested  by  Mr.  Edward  Pennock  and  described  on 
a  preceding  page.  Then  with  the  eye-piece  again  in 
position,  the  field  should  appear  brightly  lighted  in 
every  part.  It  will  probably  be  too  bright,  and  the  ob- 
server must  either  rack  down  the  condenser,  if  he  uses 
one,  until  the  desirable  softness  of  illumination  is  ob- 
tained, or  the  same  result  is  to  be  worked  for  by  rota- 
ting the  diaphragm. 

With  very  low-power  objectives  where  the  actual 
field  is  comparatively  large,  the  apparent  field  cannot 
be  evenly  lighted  by  either  mirror.  With  all  low 
powers,  and  without  a  sub-stage  condenser  to  modify 
the  intensity,  it  is  better  to  use  the  plane  mirror.  This 
will  sufficiently  illuminate  the  field  of  all  below  the  one 
inch,  until  we  reach  those  very  low  powers  now  used, 
such  as  Zeiss's  variable  A*  which  may  be  used  as  a 
three  or  a  five  inch,  with  all  intermediate  amplifications, 
by  simply  turning  an  adjustment  collar.  The  field  of 
these  lenses  cannot  be  lighted  throughout  its  whole  ex- 


MICROSCOPICAL    PRAXIS.  73 

tent  by  either  mirror.  The  concave  gives  only  a  cen- 
tral bright  spot  while  the  plane  surface  does  little  better. 
To  light  this  large  region  fully,  the  plane  mirror  should 
be  covered  with  a  disk  of  white  card-board,  and  parallel 
rays  thrown  on  it  by  the  bull's-eye  condensing  lens. 
The  mirror  and  the  card  are  then  manipulated  as  the 
mirror  alone  should  be.  Or  a  more  successful  result 
may  perhaps  be  attained  by  taking  the  light  with  the 
mirror  from  the  brightly  illuminated  inner  surface  of  a 
white  lamp-shade.  With  the  two-inch  or  the  one-inch 
the  plane  mirror  alone  is  sufficient.  All  higher  powers 
demand  the  use  of  the  concave  surface,  unless  the  bull's- 
eye  lens  is  placed  between  the  lamp  and  the  mirror, 
with  a  sub-stage  condenser  between  the  mirror  and  the 
object;  or  unless  some  form  of  microscopical  lamp  is 
used  with  a  permanent  bull's-eye  lens,  in  which  cases 
the  plane  surface  should  be  turned  toward  the  con- 
denser. 

It  sometimes  happens  that  if  the  microscope  has 
travelled  for  some  distance  by  railroad,  the  owner  will 
have  trouble  to  get  rid  of  a  broad,  crescentic  shadow  in 
an  otherwise  well-lighted  field.  No  manipulation  of 
the  diaphragm  or  other  sub-stage  appliance  serves  to 
remove  the  annoyance;  it  remains  partly  eclipsing  the 
field  by  its  region  of  darkness,  and  the  beginner 
may  think  that  something  serious  has  happened., 
The  trouble  is  caused  by  the  jarring  of  the  dia- 
phragm in  the  body  or  in  the  draw-tube,  the  journey 
having  shaken  it  too  far  downward.  Remove  the  body, 
first  of  course  taking  off  the  objective  and  the  eye-piece, 
and  push  the  diaphragm  up  a  little  way,  repeating  the 
operation  if  not  at  first  successful. 

It  is  well  for  the  observer  to  keep  both  eyes  open 
when  using  the  microscope,  and  if  he  begin  with  this 


74  MICROSCOPICAL    PRAXIS. 

plan  he  will  be  doing  well,  for  it  is  somewhat  of  a 
trouble  at  the  start  to  see  anything  in  the  microscope 
with  both  eyes  open,  as  the  unemployed  organ  seems 
to  dominate.  At  first  the  images  of  the  object  on  the 
table  and  of  the  object  under  the  microscope  will 
mingle,  but  as  combination  is  impossible,  the  result  will 
be  amusing  and  annoying.  At  one  moment  the  magni- 
fied image  will  have  the  mastery,  at  the  next  the  micro- 
scopic objects  will  dominate  the  view,  and  at  times  the 
eye  will  fail  to  take  cognizance  of  anything.  But  with 
a  little  practice  the  result  is  entirely  satisfactory,  and 
the  brain  will  finally  take  notice  of  the  magnified  image 
only. 

It  has  been  suggested  that  instead  of  allowing  the 
unoccupied  eye  to  roam  about  aimlessly  as  it  does,  and 
as  may  be  noticed  when  another  person  is  at  the  micro- 
scope with  both  eyes  open,  it  would  be  better  to  give 
it  a  dark  surface  to  gaze  at,  or  as  some  recommend,  a 
white  surface.  Consequently  many  forms  of  eye-shades 
have  been  devised.  They  are  all  applied  to  or  near  the 
eye-piece,  a  projecting  arm  carrying  a  disk  for  the  pro- 
tection of  the  unoccupied  eye.  Dr.  L.  B.  Hall,  of 
Philadelphia,  has  described  a  device  for  this  purpose 
which  may  be  made  by  the  microscopist  himself. 

It  consists  of  a  small,  opaque  disk  supported  by  a 
wire  extending  from  its  outer  edge  downward  to  a  point 
on  the  body-tube  low  enough  to  be  out  of  the  way  of 
the  nose,  then  bent  upward  parallel  to  the  tube,  but 
not  touching  it,  and  attached  to  a  ring  near  the  top. 
Dr.  Hall's-  own  eye-shade  was  made  of  No.  18  brass 
wire  about  twenty  inches  long,  with  a  loop  about  one 
and  three-quarter  inches  in  diameter  made  at  one 
end  and  covered  with  black  paper  to  form  the  disk. 
The  other  end  of  the  wire  was  made  into  a  ring  to  fit  the 


MICROSCOPICAL    PRAXIS.  75 

body-tube,  and  the  intermediate  portion  bent  as  de- 
scribed. The  ring  about  the  body-tube  may  be 
covered  with  chamois-skin  if  desired,  to  protect  the  lac- 
quer. 

To  return  the  microscope  to  the  case  demands  move- 
ments the  reverse  of  those  used  to  take  it  out.  The 
body  is  racked  up,  the  objective  unscrewed  (taking 
care  not  to  drop  it  or  to  strike  it  against  any  hard  sub- 
stance), then  gently  replaced  in  its  brass  box  and  the 
lid  screwed  on.  The  slide  is  taken  from  the  stage,  and 
if  permanently  mounted,  is  returned  to  its  proper  cabi- 
net; the  mirror  is  turned  at  right-angles  to  the  optic 
axis  of  the  instrument;  the  eye-piece  is  removed  and 
deposited  in  the  receptacle  prepared  for  it;  the  body  is 
racked  down,  and  placed  in  a  vertical  position,  and  the 
stand  is  lifted  into  the  case,  where  it  remains  with  its 
front  looking  at  the  back  wall  of  the  box.  The  door  is 
then  closed  and  locked.  If  there  are  children  around 
that  cannot  be  controlled,  the  key  would  better  be  car- 
ried in  the  owner's  pocket.  Two  minutes'  brutal  usage 
will  do  more  injury  to  a  good  stand  than  months  of 
proper  treatment.  No  one  but  the  microscopist  should 
ever  touch  the  objectives,  and  he  should  carefully  avoid 
the  contact  of  his  fingers  with  the  lens.  Optical  glass 
is  soft  and  easily  injured.  The  owner  would  do  well  to 
act  accordingly,  for  a  scratched  or  broken  objective  is 
a  ruined  one. 

Dust  is  an  insidious  and  dangerous  enemy  to  the  mi- 
croscope. It  gets  into  the  bearings  and  the  movable 
parts,  and  harms  them.  The  microscopist  should 
therefore  often  wipe  the  stand  with  a  soft  old  handker- 
chief or  a  fine  chamois-skin.  The  latter  is  the  better, 
since  the  fibres  from  the  handkerchief  often  become  a 
source  of -trouble.  No  liquid  should  be  used;  alcohol 


76  MICROSCOPICAL    PRAXIS. 

should  especially  be  avoided  as  it  will  remove  the 
lacquer.  The  rack  and  the  pinion-bearings  may  be  oc- 
casionally and  sparingly  touched  with  the  porpoise-oil 
used  by  jewellers,  or  with  very  superior  sewing- 
machine-oil.  Exceeding  small  quantities  must  be  used, 
and  the  parts  at  once  wiped  almost  dry,  otherwise  the 
oily  surfaces  will  collect  the  dust,  and  the  last  state  of 
that  microscope  will  be  worse  than  the  first.  The 
dealers  use  soft  and  thick  grease  for  lubricating  pur- 
poses, or  a  refined  tallow.  The  parts  of  a  well-made 
stand  need  lubricating  only  at  very  long  intervals. 
They  will  work  well  if  kept  perfectly  clean  and  free 
from  dust. 

The  microscopist  will  sometimes  be  annoyed  by  one 
or  more  indefinite  specks  or  spots  apparently  in  the 
field,  but  whether  on  the  eye-piece  or  on  the  objective 
he  cannot  decide.  That  they  are  not  on  the  object  or 
on  the  cover-glass  is  determined  by  moving  the  slide; 
if  the  specks  remain  stationary  they  are  either  on  the 
ocular  or  on  the  objective.  To  discriminate  between 
these,  rotate  the  ocular;  if  the  offending  particles  move 
they  are  on  it;  if  not,  they  are  on  the  objective.  If  on 
the  former,  the  two  lenses  may  be  unscrewed  and  care- 
fully wiped  by  the  Japanese  filter-paper;  if  upon  the 
objective,  the  front  lens  may  be  guardedly  touched  with 
the  Japanese  paper,  or  the  back  combination  very 
cautiously  swept  with  a  soft  camel's-hair  brush.  This 
must  be  carefully  done,  and  the  brush  must  be  scrupu- 
lously clean,  otherwise  the  glass  may  be  scratched.  In  all 
these  operations  the  breath  is  a  useful  and  harmless 
assistant. 

There  are  often  found  on  a  slide  three  objects  that 
perplex  the  beginner.  These  are  air-bubbles,  oil-drops 
and  that  quivering  and  apparent  dancing  of  minute 


MICROSCOPICAL    PRAXIS.  77 

particles  called  the  Brownian  movement,  or  pedesis,  or 
sometimes  the  pedetic  motion.  Air-bubbles  have  been 
described  as  wonderful  things.  I  remember  to  have 
once  shown  a  slide  of  urinary  deposit  to  a  physician, 
who  immediately  cried  out  that  the  patient  must  be  in  a 
dreadful  condition,  for  those  big,  round,  black-bordered 
things  surely  must  be  deadly.  Like  the  majority  of 
persons  unaccustomed  to  microscopical  investigation, 
he  had  gazed  at  the  most  prominent  object  in  the  field, 
and  not  at  what  I  wanted  him  to  see,  for  he  had  looked 
at  two  or  three  air-bubbles,  that  are  of  a  truth  rather 
frightful  to  the  uninitiated.  In  such  cases  an  indicator 
in  the  eye-piece  is  a  useful  contrivance. 


Air-Bubbles   and    Oil-Bubbles.     The   Brownian 
Movement. 

Rather  than  take  up  space  by  an  attempted  descrip- 
tion of  air-bubbles,  I  recommend  the  reader,  if  he  be 
not  already  familiar  with  them,  to  follow  the  plan  sug- 
gested by  Prof.  S.  H.  Gage  in  his  "Notes  on  Micro- 
scopical Methods,"  and  make  bubbles  for  examination. 

Air-bubbles  and  oil-drops  both  appear  with  a  bright 
central  spot  and  a  broad  dark  border.  To  obtain  the 
former,  Prof.  Gage  suggests  that  a  drop  of  mucilage 
shall  be  placed  on  a  glass  slip  and  beaten  with  a  broad 
blade  until  it  looks  milky  on  account  of  the  inclusion 


78  MICROSCOPICAL    PRAXIS. 

of  air.  Put  on  a  cover  glass  but  do  not  press  it  down. 
With  this  under  the  objective,  focus  upward  and  down- 
ward, noticing  that  in  focussing  up,  the  central  bright 
spot  becomes  very  clear  and  the  black  ring  very 
sharply  defined,  until  the  whole  is  dimmed  by  being  far 
beyond  the  focus.  As  Prof.  Gage  also  says,  the  air- 
bubble  is  one  of  the  most  useful  means  for  ascertaining 
whether  or  not  the  illumination  is  strictly  central;  if  it 
is  not,  the  bright  spot  will  not  be  in  the  centre  of  the 
bubble,  as  it  will  be  if  the  light  is  strictly  axial.  And  if 
the  mirror  be  swung  to  one  side  so  as  to  make  the  illu- 
mination oblique,  the  bright  spot  will  appear  on  the 
side  of  the  bubble  away  from  the  mirror.  This  is  an 
important  experiment  to  make  whenever  in  doubt  in 
this  connection,  as  precisely  the  opposite  effect  obtains 
in  the  oil-drop,  the  bright  spot,  when  oblique  light  is 
used,  then  being  on  the  same  side  with  the  mirror.  Oil- 
drops  may  be  prepared  for  examination  by  beating 
together  a  drop  of  mucilage  and  one  of  clove  oil.  An 
air-bubble  has  been  described  as  a  cancer-cell,  and  as  the 
microbe  of  la  grippe. 

The  reader  may  have  more  trouble  in  experimenting 
with  oblique  light  than  with  central.  About  all  that  I 
can  tell  him  is  to  swing  the  mirror  to  one  side,  usually 
toward  the  right-hand  as  being  more  convenient,  and 
then  to  manipulate  it  until  the  light  is  properly  re- 
flected on  the  object,  the  degree  of  obliquity  of  course 
varying  with  the  position  of  mirror-bar,  the  mirror  or 
both.  Frey  in  his  work  entitled  "The  Microscope  and 
Microscopical  Technology,"  says  that  "Considerable 
practice  is  requisite  with  oblique  illumination.  The 
aperture  of  the  stage  must  be  freed  from  diaphragms, 
or  any  other  apparatus  that  may  be  under  the  stage, 
and  the  various  positions  of  the  mirror  are  to  be  tried 


MICROSCOPICAL    PRAXIS.  79 

while  the  eye  is  looking  into  the  microscope.  Truly 
diabolical  illumination  is  thus  sometimes  obtained, 
which,  however,  shows  many  fine  details  in  an  astonish- 
ing manner."  Oblique  light,  as  previously  remarked, 
is  chiefly  used  in  the  resolution  of  the  fine  lines  on  the 
surfaces  of  diatoms,  these  little  ridges  not  being  too 
minute  to  cast  a  shadow  on  the  side  opposite  to  that 
from  which  the  light  is  received.  Oblique  light  is  oc- 
casionally used  to  produce  delicate  shadows  when  the 
microscopist  is  studying  other  objects,  yet  the  effect  is 
rarely  employed.  In  such  cases  however,  when  the 
mirror  is  swung  far  to  one  side,  and  the  objective  is 
not  properly  corrected,  the  result  is  as  Dr.  Frey  calls  it, 
truly  diabolical. 

The  Brownian  movement,  or  pedesis,  is  that  continu- 
ous quivering  or  dancing  common  to  all  minute  parti- 
ticles  when  suspended  in  water.  It  is  not  an  evidence 
of  life,  and  should  not  be  mistaken,  as  it  is  apt  to  be 
mistaken  by  the  beginner,  for  minute  living  creatures. 
It  may  be  seen  to  good  advantage  by  rubbing  up  a 
little  gamboge  or  India  ink  in  water,  allowing  the  larger 
parts  to  settle,  and  then  examining  a  drop  of  the  super- 
natant liquid  with  a  high-power  objective.  The  field 
will  be  full  .of  dancing  and  trembling  particles,  moving 
irregularly,  but  as  if  endowed  with  life.  Similar  move- 
ments are  beautifully  visible  within  certain  desmids  and 
algae,  especially  if  they  are  not  in  a  healthy  condition, 
the  minute  black  granules  there  hovering  together, 
swinging  and  quivering  like  a  swarm  of  microscopic 
bees.  It  is  also  noticeable  within  the  little  sacs  near 
the  base  of  the  spinal  nerves  of  the  common  frog,  and 
in  almost  any  place  where  finely  divided  matters  are  in 
suspension  in  a  thin  liquid.  The  cause  of  the  move- 
ment is  not  known.  It  has  been  supposed  to  be  pro- 


8o  MICROSCOPICAL    PRAXIS. 

duced  by  currents  of  heat  or  of  electricity,  but  the  sub- 
ject is  still  without  a  satisfactory  explanation.  How 
long  it  lasts  is  also  unknown.  One  writer  claims  to 
have  prepared  a  slide  which  he  examined  at  the  end  of 
seven  years,  and  found  the  particles  as  active  as  at  first. 
Soap  and  water  are  said  to  produce  an  energetic  pede- 
sis,  and  it  is  claimed  that  our  hands  are  cleansed  as 
effectually  by  the  violent  Brownian  movements  of  the 
soap;. as  by  its  chemical  action.  In  any  event,  do  not 
mistake  this  uncertain  dancing,  as  seen  under  the  mi- 
croscope, for  the  quivering  of  minute  bacteria  or  for 
any  other  living  plants  or  animals. 

In  addition  to  the  three  foregoing  microscopical  bug- 
bears, which  cease  to  terrify  when  well  known,  the 
student  should  make  himself  familiar  with  the  appear- 
ances of  starch-granules,  and  with  cotton,  woolen  and 
linen  fibres,  especially  when  colored,  as  woolen  fibres 
are  likely  to  be  if  the  work-room  is  carpeted,  as  it  should 
not  be.  I  can  scarcely  imagine  an  object  any  more  as- 
tonishing on  first  acquaintance  than  a  purple  fibre  from 
the  carpet  or  elsewhere.  These,  with  cotten  and  linen, 
are  likely  to  be  found  in  any  preparation;  even  in 
mounted  slides  they  are  common,  having  fallen  into  the 
mounting  medium  or  been  entangled  in  the  object  it- 
self. The  student  may  mistake  them  for  something 
more  important,  unless  he  makes  himself  familiar  with 
them  at  the  beginning  of  his  studies.  Cotton  fibres 
have  been  mistaken  for  casts  from  the  uriniferous 
tubules  of  the  human  kidney. 

The  difference  in  the  appearance  of  convex  and  con- 
cave bodies  is  also  important  and  useful.  Microscopi- 
cal and  ordinary  vision  differ  so  widely  from  each 
other,  that  it  is  often  impossible  to  decide  whether  a  sur- 
face is  convex  or  concave,  especially  if  the  object  be 


MICROSCOPICAL    PRAXIS.  8l 

uniformly  covered  with  markings  that  may  be  bosses  or 
depressions;  sometimes  the  same  trouble  is  experienced 
by  the  naked  eye.  Grooves  belong  in  the  same  cate- 
gory with  depressions.  When  the  objective  is  racked 
upward,  a  convex  surface  will  appear  lighter;  a  concav- 
ity will  appear  lighter  when  the  objective  is  focussed 
downward. 

It  is  often  difficult  to  find  a  small  object  and  bring  it 
into  the  field.  This  is  especially  true  with  high  powers, 
the  trouble  increasing  with  the  increase  of  magnifica- 
tion, because  the  actual  field  of  high-power  objectives  is 
so  small  that  the  chances  of  escape  for  the  small  'object 
are  much  greater  than  are  the  microscopist's  for  cap- 
turing it,  especially  if  the  mechanical  stage  be  not  used. 
The  only  recourse  is  to  remove  the  high-power  object- 
ive, substitute  a  lower  power,  find  the  object,  place  it  in 
the  centre  of  the  field,  and  then  to  re-attach  the  high- 
power  lens,  when  the  object  sought  should  be  some- 
where within  the  illuminated  space. 


Thin  Glass. 

Before  thin  glass  was  obtainable  as  easily  and  cheaply 
as  at  present,  microscopists  used  very  thin  pieces  of 
mica,  and  for  use  with  exceedingly  high-power  object- 


82  MICROSCOPICAL    PRAXIS. 

ives  whose  working-distance  is  excessively  short,  it  is  to 
a  certain  extent  still  used.  The  older  microscopists 
preserved  their  objects  between  two  pieces  of  window- 
glass. 

The  method  of  manufacturing  this  thin  microscopi- 
cal glass  is  a  secret  known  only,  I  believe,  to  the 
Messrs.  Chance  of  Birmingham,  England.  Some 
time  ago  the  report  passed  the  rounds  of  the  journals  to 
the  effect  that  the  method  of  making  it  had  been  dis- 
covered in  Germany,  and  that  it  would  soon  be  sup- 
plied very  cheaply,  but  nothing  further  has  been  heard 
from  it  of  late.  All  that  is  used  in  this  country  is  im- 
ported in  sheets  and  cut  by  the  dealers  into  circles  or 
squares  of  various  sizes.  A  suggestive  remark  in  this 
connection  is  made  by  Dr.  S.  Czapski  when  writing  of 
the  peculiar  cover-glass  needed  for  use  with  Zeiss's 
latest  apochromatic  objective  of  1.63  N.  A.  He  says, 
"The  production  of  these  cover-glasses  in  the  usual  way 
— by  blowing  in  a  furnace — was  forbidden  by  their  sub- 
stance." Further  than  this  the  method  of  manufacture 
has  not  been  explained. 

The  thin  sheets  of  glass,  so  brittle  that  they  break 
almost  at  a  word  or  a  look,  are  .cut  with  a  diamond. 
The  circles  are  made  by  placing  on  the  glass  a  plate 
pierced  with  holes  somewhat  larger  than  the  disks  de- 
sired, and  a  diamond  is  then  run  around  inside  these 
openings  until  the  entire  surface  of  the  glass  sheet  is 
covered  with  the  scratched  circles.  To  attempt  to 
break  these  out  would  be  followed  by  disastrous  conse- 
quences, but  if  the  glass  be  laid  aside  for  a  day  or  two, 
the  disks  will  fall  out  of  their  own  accord.  The  squares 
awre  cut  with  a  diamond  and  a  ruler,  after  the  thin  sheet 
has  been  attached  by  water  to  a  flat  surface,  usually  of 
plate  glass.  After  the  cutting  the  squares  are  easily 


MICROSCOPICAL    PRAXIS.  83 

broken  off  by  sliding  the  sheet  to  the  edge  of  the  sup- 
port. 

When  the  glass  comes  from  the  dealer  it  is  never 
clean  enough  to  be  used  for  microscopical  purposes, 
but  to  clean  it  sufficiently  nothing  more  is  usually 
needed  than  a  careful  wiping  between  two  surfaces  of 
soft,  old  muslin.  I  have  never  found  it  necessary  to 
use  any  of  the  chemical  mixtures  recommended  by  some 
authors. 

Several  mechanical  cleaning-devices  have  been  de- 
scribed, but  nothing  is  better  than  two  smooth,  wooden 
blocks,  each  with  a  surface  tightly  and  smoothly  cov- 
ered with  soft,  thick  chamois-skin.  The  cover-glass  is 
placed  on  one  block  while  the  other  rubs  the  surface 
until  it  is  clean,  when  the  whole  is  turned  over  and  the 
other  block  rubs  the  other  side  of  the  glass.  With  this 
simple  contrivance  it  is  hardly  possible  to  break  the 
thinnest  cover. 

After  being  cleaned  the  glass  should  be  kept 
clean,  as  well  as  free  from  scratches.  It  is  ex- 
posed to  the  latter  if  thrown  loosely  into  a  box, 
where  a  particle  of  hard  dust  may  do  mischief. 

Mr.  C.  E.  Hanaman  of  Troy,  N.  Y.,  has  described  a 
method  of  keeping  the  covers  protected  from  accident 
and  from  dirt.  He  places  them  in  drawers  or  in  boxes 
filled  with  narrow  strips  of  new  white  blotting-paper, 
between  which  they  are  stood  on  edge.  This  method, 
Mr.  Hanaman  says,  not  only  preserves  them  from  break- 
age and  enables  him  readily  to  pick  them  out  when 
wanted  for  use,  but  also  assists  him  to  select,  for  special 
preparations,  those  of  the  most  desirable  thickness,  as, 
by  holding  the  drawer  or  the  box  between  the  eye  and 
the  light,  it  is  easy  to  select  the  thickest  or  the  thinnest. 

The  glass  varies  a  good  deal  in  thickness  even  in  the 


84  MICROSCOPICAL    PRAXIS. 

same  lot  as  supplied  by  the  optician.  Its  thickness  may 
be  most  conveniently  measured  by  some  sort  of  microm- 
eter-gauge, of  which  there  are  several  in  the  market 
for  the  use  of  machinists.  Carl  Zeiss  makes  one  espec- 
ially for  the  microscopists,  but  either  of  the  three  man- 
ufactured in  this  country,  especially  that  of  Bausch  and 
Lomb,  will  answer  equally  well.  It  is  not  only  conven- 
ient but  important  to  have  covers  of  different  thick- 
nesses sorted  out,  so  that  almost  any  kind  may  be  had 
at  a  moment's  notice,  and  the  micrometer-gauge  is 
slightly  more  accurate  in  its  results  than  the  fine-ad- 
justment screw,  with  which  the  thickness  may  also  be 
estimated.  An  experiment  made  while  writing  this 
gives  the  thickness  of  a  cover  as  measured  by  a  gauge 
to  be  one  two-hundredth  inch,  while  the  fine-adjust- 
ment screw  gives  0.0045,  or  about  -g^-g-  inch. 

To  use  the  former,  place  the  cover  between  the  jaws 
of  the  instrument,  close  them  and  read  the  thickness  in 
hundredths  and  thousandths  of  an  inch.  To  use  the 
fine-adjustment  screw  for  the  purpose,  if  the  milled 
head  be  graduated  and  the  value  of  the  spaces  known, 
it  is  only  necessary  to  count  the  number  of  divisions 
employed  in  focussing  from  one  surface  of  the  glass  to 
the  other.  A  few  particles  of  dust  will  answer  as  ob- 
jects on  which  to  focus,  or  a  minute  drop  of  ink  on 
each  side  of  the  cover,  and  so  close  as  to  be  together 
in  the  field  of  a  high-power  objective,  but  not  close 
enough  to  overtop  each  other.  In  the  experiment  re- 
ferred to,  the  number  of  the  divisions  on  the  milled 
head  used  in  focussing  from  the  lower  to  the  upper  sur- 
face 'of  the  cover,  were  four  and  one-half,  and  as  each 
division  corresponds  to  a  movement  of  the  body-tube 
of  one  one-thgusandth  inch,  the  cover  was  four  and  one- 
half  thousandths,  or  ^4^  inch  thick. 


MICROSCOPICAL    PRAXIS.  85 

The  most  desirable  as  well  as  the  cheapest  and  best 
micrometer-gauge  now  in  the  market  is  made  by 
Messrs.  Bausch  and  Lomb  of  Rochester,  N.  Y.  This 
instrument  possesses  several  commendable  features 
which  may  become  of  great  importance  to  the  working 
microscopist. 

If  the  reader  have  no  graduated  milled-head  to  his 
fine-adjustment  screw,  and  no  micrometer-gauge  in  his 
possession,  he  may  still  measure  his  thin  glass,  although 
the  process  is  not  as  convenient  as  with  these  appli- 
ances. 

Place  the  cover  upright  on  the  stage  so  that  its  edge 
shall  present  toward  the  objective.  A  narrow  slit  in  a 
cork  or  even  in  a  slide  of  soft  wood  will  hold  it  in  posi- 
tion, and  enough  light  will  be  reflected  from  the  edge 
of  the  glass  to  supply  the  demand.  Focus  the  object- 
ive on  the  glass,  place  the  microscope  in  a  horizontal 
position,  apply  the  camera  lucida  to  the  eye-piece,  and 
draw  two  lines  to  represent  the  borders  of  the  magni- 
fied edge  of  the  thin  cover.  Remove  the  glass,  for  it 
substituting  a  stage-micrometer,  and  draw  one  or  more 
of  the  micrometer  spaces  on  the  paper.  As  the  value 
of  these  spaces  will,  be  known,  a  comparison  between 
them  and  the  magnified  width  of  the  glass  may  be  easily 
made,  the  two  having  been  enlarged  to  the  same  degree 
by  the  same  objective  and  eye-piece.  If  the  width  of 
the  glass  as  represented  on  the  paper  occupies  one-fifth 
of  the  micrometer-space  as  shown  on  the  paper,  the  ac- 
tual thickness  of  the  glass  will  of  course  be  one-fifth  of 
whatever  may  be  the  value  of  that  special  space  on  the 
micrometer.  If  the  distance  for  the  glass  should  equal 
the  distance  of  the  one  one-hundredth  inch  space  on 
the  micrometer,  of  course  the  thickness  of  the  glass  is 
one  one-hundredth  of  an  inch. 


86  MICROSCOPICAL    PRAXIS. 

Dr.  S.  Czapski  reports  the  following  original  method 
for  determining  the  thickness  of  the  cover-glass  over 
mounted  preparations,  a  measurement  which  it  is  often 
important  to  know  for  high-power  work.  The  proced- 
ure presupposes  the  possession  of  some  cover-glasses, 
the  thickness  of  which  is  known,  and  that  the  head  of 
the  fine-adjustment  screw  is  divided  by  radial  lines. 

The  upper  and  lower  surfaces  of  the  cover  are  fo- 
cussed  with  central  illumination,  and  the  amount  of 
turn  given  to  the  fine-adjustment  screw  noted  for  each 
cover-glass,  it  being  unimportant  whether  the  exact 
value  of  the  screw  is  known  or  not;  the  reader  therefore 
having  it  in  his  power  to  make  his  own  radial  lines  on  a 
disk  of  paper  to  be  pasted  on  the  milled  head  of  the 
fine-adjustment  screw.  If  the  surfaces  of  the  cover- 
glass  do  not  present  any  obvious  marks  to  focus  on, 
an  artificial  one,  such  as  dust  or  scratches,  must  be  sup- 
plied. If  the  numbers  thus  obtained  be  compared  with 
the  known  real  thickness  of  the  covers,  a  reduction- 
factor  is  obtained  from  their  quotients,  which  is  avail- 
able for  determining  measurements  of  a  similar  kind, 
that  is  to  say,  for  measurements  of  other  cover-glasses 
with  the  same  objective,  ocular,  diaphragm  and  tube- 
length.  The  focussing  differences  are  always  to  be 
multiplied  by  this  factor  in  order  to  obtain  the  true 
thickness  of  the  layer. 

As  an  example: — Objective  DD  Zeiss,  diaphragms 
mm.  in  diameter;  tube  length  155  mm.;  and  four  cover- 
glasses,  the  thickness  of  which,  already  ascertained,  are 
0.146,  0.168,  0.187,  0.22.  The  focussing  differences 
marked  by  the  head  of  the  fine-adjustment  screw  were 
35,  40,  45,  52  divisions.  Then  the  reduction-factors  in 
i/ioooyu  are  ^  _  4>I7;  y_8  =  4.2o;  yy  =  4-l6;  %o 
—  4.23;  or  on  the  average  4.19,  say  4.20.  If  the  thick- 


MICROSCOPICAL    PRAXIS.  87 

ness  of  these  cover-glasses  had  not  been  known,  but  the 
focussing  difference  had  been  obtained  and  multiplied 
by  4.2,  the  results  would  have  been  0.147,  0.168,  0.189, 
0.218,  instead  of  0.146,  0.168,  0.187,0.22.  Differences 
of  -f-  o.ooi,  o.o,  -f-  0.002,  —  0.002;  a  result  more  than 
sufficiently  accurate  for  the  purpose. 

Another  method  of  measuring  the  cover-glass  of  a 
permanently  mounted  preparation  by  means  of  the  ad- 
justment-collar of  the  objective,  has  been  suggested  by 
Professor  C.  K.  Wead.  This  also  necessitates  the  poss- 
ession of  a  cover  whose  thickness  is  accurately  known. 
With  the  adjustment-collar  placed  at  zero  (the  open 
point),  dust,  finger-marks  or  other  minute  objects  are 
focussed  on  the  lower  surface  of  the  cover-glass  whose 
thickness  is  known,  and  the  collar  is  turned  until  simi- 
lar objects  are  in  focus  on  the  upper  surface.  This 
should  be  repeated  several  times  and  a  record  kept  of 
the  number  of  spaces  through  which  the  collar  has  been 
turned;  take  the  average  number  of  spaces  and  divide  it 
into  the  known  thickness  of  the  cover.  The  quotient 
will  be  the  number  which  should  be  used  as  a  multi- 
plier of  the  spaces  on  the  collar  when  it  is  used  in  the 
foregoing  way  to  ascertain  the  thickness  of  a  cover- 
glass  on  a  mounted  slide. 

An  example  will  make  this  plain.  Suppose  the 
known  thickness  to  be  0.0058  inch  and  several  readings 
of  the  collar  to  be  3°. 6,  3°. 75,  &c.,  the  average  of  all  the 
experiments  being  3.56.  Dividing  3.56  into  0.0058  (the 
known  thickness  of  the  cover  used  to  obtain  the  value  of 
the  spaces  on  this  special  objective),  the  quotient  is 
0.001629,  which  is  hereafter  to  be  used  as  the  multiplier 
of  the  spaces  utilized  over  some  cover  of  unknown 
thickness.  If  we  assume,  for  the  sake  of  the  example, 
that  the  number  of  spaces  is  7.50,  multiplying  this  by 


88  MICROSCOPICAL    PRAXIS. 

the  decimal  fraction  mentioned,  we  have  0.0124175,  the 
thickness  sought  for  the  cover  of  the  mount. 

This  convenient  method  may  be  verified  by  the  grad- 
uated milled-head  of  the  fine-adjustment  screw. 

It  is  exceedingly  important  to  know  the  thickness  of 
the  cover  when  using  non-adjustable  objectives.  These 
lenses  are  corrected  by  their  makers  for  a  certain  thick- 
ness, and  as  the  adjustment  cannot  be  changed,  and 
since  the  microscopist,  until  recently,  has  not  known 
for  what  cover-thickness  the  objectives  were  intended, 
he  was  at  the  mercy  of  circumstances,  and  forced  to  ac- 
cept and  be  content  with  whatever  image  he  could  get. 
Now  we  are  able  to  use  covers  of  at  least  approximate 
correctness  with  our  non-adjustable  objectives,  thanks 
to  Prof.  S.  H.  Gage,  of  Cornell  University,  who  has  in- 
vestigated the  matter.  Prof.  Gage  submitted  certain 
questions  in  relation  to  the  subject  to  the  various  opti- 
cians of  the  world,  and  from  his  published  results  I 
take  the  following,  so  that  the  reader,  if  he  possess 
non-adjustable  objectives  by  any  of  these  makers,  may 
use  covers  of  the  proper  thickness  to  obtain  the  best 
results,  provided  his  microscope  body-tube  is  of  the 
standard  length. 

f  J.  Grunow,  New  York. 
fifa  mm.       }  H.  R.  Spencer  and  Smith,  Buffalo,  N.  Y. 

(  Powell  &  Lealand,  London. 
1 6.-15  mm       Ross  &  Co.,  London. 

mm.          Bausch  &  Lomb,  Rochester,  N.  Y. 

mm.      Carl  Zeiss,  Jena. 
Tt.6_  mm.          Zeiss  for  his  Apochromatic  oil  immersions. 
^  j  The  Gundlach  Optical  Co.,  Rochester,  N.Y. 

(  R.  &  J.  Beck,  London. 
11=1.7  mm.      J.  Zentmayer,  Philadelphia. 
^  mm.  Nachet  et  Fils,  Paris. 


MICROSCOPICAL    PRAXIS.  89 

mm.         Swift  &  Son,  London. 

mm.  C.  Reichert,  Vienna. 
The  glass,  which  is  said  to  be  crown-glass,  from 
which  the  covers  are  made,  is  brittle,  and  until  the 
microscopist  becomes  somewhat  of  an  expert  he  will 
break  them  with  amazing  facility.  The  skill  needed  in 
handling  the  fragile  things  is  easily  acquired,  but 
reasoning  from  the  number  of  devices  recommended 
for  the  purpose,  their  inventors  have  despaired  of  ac- 
quiring that  skill  themselves,  and  have  judged  others 
by  their  own  standard.  Many  and  peculiar  cover-glass 
forceps  are  obtainable,  all  more  or  less  useful,  perhaps, 
but  I  have  never  tried  them,  having  always  relied  on 
my  fingers  alone.  The  simplest  device,  and  one  readily 
made  by  any  novice,  is  the  following,  recommended  by 
Mr.  J.  C.  Douglas,  who  says  that  he  long  wanted  "a 
simple  appliance  for  picking  covers  out  of  the  liquid  in 
which  they  may  be  soaking,  selecting  them  from  their 
box,,  placing  them  flat  upon  the  object  to  be  examined 
or  mounted,  and  picking  them  off  the  slide  when  neces- 
sary after  examining  the  object  covered.  Forceps  and 
needles  have  grave  inconveniences.  Chase's  mounting 
forceps  simply  drop  the  cover,  and  are  inferior,  both 
in  simplicity  and  utility,  to  the  following  plan:  Cut  a 
piece  of  suitable  size  from  a  flat  rubber  ring;  fix  this, 
by  a  large-headed  pin  cut  short,  on  to  the  end  of  a  ce- 
dar stick,  driving  the  head  of  the  pin  so  as  to  form  a 
depression  in  the  rubber;  wet  the  rubber,  and  on  press- 
ing it  against  a  cover-glass  it  will  adhere  to  it,  and  the 
glass  may  be  manipulated  as  desired.  To  disconnect 
the  rubber  from  the  glass,  it  is  merely  necessary  to  in- 
cline the  stick  so  as  to  detach  the  rubber  at  one  edge, 
when  the  adhesion  ceases  at  once.  The  apparatus  is 
more  durable  if  a  little  cementing  material  be  used  on 


90  MICROSCOPICAL    PRAXIS. 

the  stick,  as  the  pin  sometimes  draws  through  the  rub- 
ber." 

Personally  I  prefer  to  get  along  without  any  other 
help  than  a  fine  needle  in  a  match  handle,  using  the 
needle  to  lift  the  cover  so  as  to  take  it  in  the  fingers, 
and  also  as  a  means  to  lower  it  slowly  over  the  object 
after  having  placed  one  edge  against  the  slide,  support- 
ing the  opposite  margin  by  the  needle.  In  this  way 
the  cover  may  be  gradually  depressed  or  as  slowly 
raised. 


The  Objective. 

The  objective  is  the  combination  of  lenses  applied  to 
the  lower  end  of  the  body-tube,  and  so  named  because, 
when  in  use,  it  is  near  the  object  to  be  examined.  It 
produces  an  enlarged  image  of  the  object,  which  is  in 
its  turn  enlarged  by  the  eye-piece,  the  optical  combin- 
ation thus  forming  the  compound  microscope. 

When  the  optician  has,  with  infinite  labor,  accurately 
ground  the  minute  bits  of  glass  which  he  is  forced  to 
use  in  his  high-power  objectives,  when  he  has  care- 
fully placed  them  in  the  brass  mounting  so  that  the 
centre  of  each  lens  shall  be  precisely  over  the  centre  of 
all  the  other  lenses  below  it,  and  when  he  has  corrected, 
as  well  as  he.  may,  the  chromatic  and  the  spherical 
aberrations,  he  finds  that  the  cover-glass  which  the  mi- 
croscopist  places  over  his  objects,  is  the  cause  of 
further  trouble. 


MICROSCOPICAL    PRAXIS.  91 

Correcting  the  Objective  for  the  Cover-glass. 

An  objective  that  will  produce  a  fine  image  of  the 
object  without  a  cover-glass,  that  is,  one  corrected 
for  an  uncovered  object,  will  give  an  imperfect  image 
when  used  over  covered  specimens.  This  effect  was 
first  noticed  by  Andrew  Ross,  one  of  the  older  British 
opticians,  who  also  discovered  the  means  of  correcting 
it,  this  being  done  by  separating  or  approximating  the 
front  lens  and  the  back  systems  of  lenses  which  to- 
gether form  the  objective.  The  possession  of  this  means 
for  cover-glass. correction  makes  the  difference  between 
an  adjustable  and  a  non-adjustable  objective.  To  the 
former  the  optician  adds  a  rotating  collar,  by  the  move- 
ment of  which  the  microscopist  alters  the  position  of 
the  front  lens,  or  of  the  back  systems,  and  thereby  ad- 
justs the  objective  for  the  thickness  of  the  cover.  The 
non-adjustable  objectives  are  without  the  rotating  col- 
lar or  other  means  for  altering  the  fixed  position  of  the 
lenses,  and  these  forms  are  therefore  corrected  by  the 
maker  for  a  certain  thickness  of  cover,  and  are 
immovably  adjusted  at  that  point,  so  that 
the  use  of  a  thinner  or  of  a  thicker  glass  interferes  with 
the  corrections  and  produces  an  imperfect  image.  The 
thickness  for  which  certain  opticians  adjust  these  kinds 
of  objectives  has  already  been  given,  as  ascertained  by 
Prof.  S.  H.  Gage,  and  to  that  list  the  reader  is  referred, 
•so  that  with  his  non-adjustable  objectives  he  may  use 
the  cover-glass  of  the  thickness  recommended  by  the 
manufacturer. 

Low-power  objectives,  such  as  the  one-inch  and 
lower,  never  have  means  of  correcting  them  for 
the  influence  the  cover-glass,  and  such  high-powers 
as  the  one-fourth  and  the  one-fifth  are  often  without  it, 
especially  if  the  angular  aperture  is  small.  The  best 


92  MICROSCOPICAL    PRAXIS. 

high-power  objectives  always  have  the  'correction  adjust- 
ment, the  collar  in  the  cheaper  forms  moving  the  front 
lens,  whilst  in  first-class  objectives  it  acts  on  the  back 
systems,  the  front  lens  being  stationary.  The  effect  is 
the  same  in  either  case,  but  the  last-mentioned  method 
is  the  better,  since  it  obviates  all  danger  of  bringing 
the  front  lens  in  contact  with  the  cover,  to  the  proba- 
ble detriment  of  one  or  of  both,  and,  which  is  as  im- 
portant, prevents  the  possibility  of  throwing  the  lenses 
out  of  centre  as  may  sometimes  through  rarely 
happen  when  the  adjustment  is  made  by  the  front  lens. 

In  most  adjustable  objectives  the  collar  is  so  gradu- 
ated that  when  the  adjustment  has  been  obtained  for  a 
certain  cover-thickness  it  may  be  recorded,  and  re- 
peated in  the  future  without  loss  of  time.  When  the  col- 
lar is  at  zero  the  lenses  are  as  wide  apart  as  possible,  and 
the  adjustment  is  said  to  be  open,  or  at  the  open  point, 
and  it  is  then  corrected  for  an  uncovered  object.  When 
turned  as  far  as  possible  in  the  opposite  direction  the 
lenses  are  brought  nearer  together  and  are  then  said  to 
be  closed,  and  the  objective  is  corrected  for  the  thick- 
est glass  over  which  it  can  be  used.  Some  cheap  Brit- 
ish objectives  are  adjustable  by  the  sliding  back  and 
forth  of  an  outer  tube  carrying  the  front  lens,  two 
marks  on  the  brass  mounting  being  labelled  "covered" 
and  "uncovered."  But  this  method  is  very  inelegant, 
uncertain  and  inaccurate. 

The  influence  of  the  cover  is  scarcely  noticeable  even 
with  moderately  high  powers  of  small  or  of  medium  an- 
gles of  aperture.  With  wide  angles  it  becomes  a  dis- 
turbing element  of  importance,  at  least  to  the  accom- 
plished microscopist.  By  the  novice  the  effect  would 
probably  be  overlooked  even  when  using  the  best  of 
wide-angled  objectives,  yet  like  so  many  other  points 


MICROSCOPICAL    PRAXIS.  93 

in  use  of  the  microscope,  as  one's  knowledge  increases 
and  as  the  eye  becomes  educated  and  drilled  in  the  per- 
formance of  optical  feats  new  to  it  at  first,  the  observer 
will  begin  to  perceive  deficiences  in  the  image,  espec- 
ially if  he  read  of  the  fine  action  of  similar  objectives 
in  the  hands  of  others,  or  sees  that  action  through  the 
microscope  of  his  expert  friends. 

The  microscopist  must  teach  himself  by  experiment 
and  by  study  how  to  adjust  his  objective.  He  will 
often  go  wrong  if  he  be  working  alone,  for  the  proper 
results  can  be  recognized  only  after  much  experience. 
How  best  to  make  the  adjustment  must  be  worked  out 
by  the  solitary  student  slowly  and  painfully;  it  can  be 
taught  by  personal  intercourse  and  demonstration;  it 
cannot  be  taught  on  paper.  The  last  mentioned 
method  has  been  tried,  and  the  failure  was  a  dismal 
one.  Yet  there  are  certain  suggestions  which  are  val- 
uable and  helpful,  and  may  be  used  as  crutches  until 
the  novice  is  able  to  walk  alone.  If  all  opticians  would 
graduate  the  adjustment-collar,  as  some  now  do,  so  that 
the  proper  thickness  of  cover-glass  should  be  indicated 
by  the  figures  on  the  brass  mounting,  the  difficulty 
would  be  lessened,  as  we  could  then  measure  our  thin 
glass  and  adjust  accordingly. 

The  image  should  be  clear,  distinct  and  brilliant.  It 
should  not  be  misty,  semi-obscure  or  dull.  The  mar- 
gins, as  some  one  has  said,  should  be  as  sharply  defined 
as  are  the  finest  lines  in  the  best  steel-engravings,  and 
the  narrow  boundary  lines  should  be  the  narrowest  pos- 
sible and  perfectly  black.  These  appearances  depend 
greatly  on  a  quality  of  the  objective  called  its  power  of 
definition,  and  for  this  the  optician  is  responsible.  But 
in  reference  to  these  delicately  defined  outlines  a  writer 
has  said,  that  when  a  good  objective  is  correctly  ad- 


94  MICROSCOPICAL    PRAXIS. 


justed,  they  should  be  about  -n^Vo  <r  of  an  mc^  m  width. 

Every  movement  of  the  adjustment-collar  necessi- 
tates a  change  in  the  focus,  so  that  while  adjusting  the 
objective  the  microscopist  keeps  one  hand  at  the  fine- 
adjustment  screw,  and  the  other  on  the  milled  ring  of 
the  collar,  following  the  movement  of  the  latter  by  a 
counteracting  movement  of  the  fine  adjustment,  the 
collar,  at  the  beginning  of  the  work,  usually  being 
placed  at  zero,  the  open  point  of  the  lenses.  The  ob- 
jective is  then  focussed,  and  the  collar  turned  toward 
the  closed  point  until  the  best  definition  is  obtained, 
the  microscopist  altering  the  position  of  the  adjustment 
toward  open  and  closed  even  after  he  has  what  seems 
to  be  a  good  result,  so  that  he  may  improve  it  if  possi- 
ble. 

There  are  two  suggestions  in  this  connection  that 
are  exceedingly  useful  to  the  novice  and  to  the  ama- 
teur. The  one  is  made  by  Mr.  W.  H.  Wenham,  and  is 
that  the  microscopist  shall  select  any  dark  speck  or  any 
opaque  portion  of  the  object,  and  bring  the  outlines 
into  perfect  focus;  then  place  the  finger  on  the  fine-ad- 
justment screw  and  move  it  briskly  backwards  and  for- 
wards in  both  directions  from  the  first  position.  Ob- 
serve the  expansion  of  the  dark  outlines  of  the  object, 
both  when  within  the  focus,  that  is,  when  the  object  is 
nearest  to  the  objective,  and  when  it  is  without  the  fo- 
cus, or  furthest  from  the  objective.  If  the  greater  ex- 
pansion, or  "coma,"  is  when  the  object  is  without  the 
focus,  the  lenses  must  be  placed  further  asunder,  or  to- 
ward the  mark  "uncovered,"  that  is,  toward  the  open 
point  or  zero.  If  the  greater  expansion  is  when  the  ob- 
ject is  within  the  focus,  the  lenses  must  be  brought 
closer  together,  or  toward  closed  point.  When  the  ob- 
jective is  in  proper  adjustment,  the  expansion  of  the 


MICROSCOPICAL    PRAXIS.  95 

outline  is  exactly  the  same,  both  within  and  without  the 
focus.  When  the  scales  of  the  insect  called  the  Podura, 
and  also  when  diatoms  are  used  as  the  test  objects,  if 
the  dots  or  other  markings  on  the  surface  have  a  tend- 
ency to  run  into  lines  when  the  object  is  placed  with- 
out the  focus,  the  lenses  must  be  brought  closer  to- 
gether; but  on  the  contrary,  if  the  lines  appear  when 
the  object  is  within  the  focus,  the  lenses  should  be 
further  separated. 

This  is  an  exceedingly  difficult  method  in  actual  prac- 
tice, demanding  much  experience  and  more  skill  than 
is  possessed  by  even  accomplished  amateurs.  The  nov- 
ice will  soon  observe  that  for  his  purposes  the  sugges- 
tion is  worthless.  Yet  is  a  valuable  method,  and  in  the 
hands  of  an  expert  microscopist,  capable  of  satisfactory 
results.  Still,  even  for  the  expert,  there  is  a  better 
method,  that  of  adjusting  for  the  best  image,  and  it  is 
doubtful  if  the  accomplished  microscopist  ever  thinks 
of  using  any  other. 

Another,  and  for  the  beginner,  much  simpler  method 
is  that  recommended,  I  think,  by  Dr.  Lionel  S.  Beale. 
Here,  with  the  adjustment-collar  at  zero,  the  objective 
is  focussed  on  the  object,  and  the  collar  turned  until  a 
particle  of  dust,  a  striation,  or  any  minute  inperfection 
on  the  upper  surface  of  the  cover  is  brought  sharply 
into  focus,  when  the  objective  will  be  corrected  for 
that  particular  glass  and  preparation.  The  microsco- 
pist then  re-focusses  with  the  fine-adjustment  screw, 
and  proceeds  with  the  investigation.  This  however,  is 
applicable  only  to  objectives  whose  adjustment-collar 
moves  the  front  lens. 

The  Podura  already  referred  to,  and  whose  scales  are 
so  useful  to  the  microscopist  for  testing  certain,  quali- 
ties of  his  objectives,  is  an  agile  little  insect  to  be  found 


96  MICROSCOPICAL    PRAXIS. 

under  stones  and  sometimes  in  damp  cellars,  where  it 
jumps  about  with  surprising  alacrity.  Its  scales  should 
be  mounted  dry,  but  it  is  much  the  better  plan  to  buy 
a  slide  of  them  from  the  dealer,  since  the  little  creature 
is  difficult  to  capture,  and  is  in  addition,  not  very  com- 
mon. 

The  scales  are  minute  and  have  given  the  opticians 
and  the  microscopists  a  lively  time  in  the  discussion  as 
to  the  character  of  the  markings  which  the  microscope 
reveals.  These  markings  have  the  form  of  exclamation 
points  from  which  the  dot  or  period  at  the  base  has  been 
removed.  The  discussion,  which  is  even  now  unsettled, 
is  whether  the  exclamation  points  are  spines  projecting 
from  the  surface  of  the  scale  and  from  which  they  may 
be  removed  by  electricity,  as  some  contend,  or  whether 
they  are  simply  elevated  folds  or  ridges  of  that  surface. 
But  whatever  may  be  their  character,  they  form  excel- 
lent objects  over  which  to  study  the  adjustment  of  the 
objective.  When  the  objective  is  properly  adjusted 
and  focussed,  for  the  focussing  is  here  of  great  impor- 
tance, the  exclamation  marks  should  stand  out,  each 
one  distinct  from  its  neighbor  on  each  side,  and  the 
lower  part  should  not  trail  off  into  the  head  of  the 
mark  below  it,  but  should  end  clearly  and  sharply,  with 
a  well-defined  space  of  varying  extent  between  its 
termination  and  the  rounded  head  or  upper  region 
of  the  next  exclamation  point  below  it. 

Messrs.  R.  and  J.  Beck,  of  London,  issue  a  circular 
descriptive  of  their  "National  Stand/'  in  which  the 
appearances  of  the  Podura  scale  are  shown  under  the 
correct  and  incorrect  adjustments  of  the  objective, 
within  the  proper  focus  and  without  it.  These  are  here 
reproduced  in  Figs.  4-9.  In  Fig.  4.  the  scale  is  shown 
as  it  appears  when  the  objective  is  correctly  adjusted 


MICROSCOPICAL    PRAXIS. 


97 


Fig.  4. 


Fix.  5- 


Fig.  6. 


Fig.  7. 


Fig.  8. 


Fig.  9. 


Podura  Scale. 


Fig.  4,  as  the  scale  should  appear,  the  objective  being  correctly  adjusted  and 
focussed.  Fig.  5,  the  same  as  it  appears  on  each  side  of  the  focus,  when  the 
objective  is  correctly  adjusted.  Fig.  6,  here  the  adjustment  is  correct,  but  the 
focus  is  slightly  wrong.  Fig.  7,  the  objective  is  incorrectly  adjusted,  but  properly 
focussed.  Fig.  8,  the  same  beyond  the  focus.  Fig.  9,  the  same  within  the  focus. 


98  MICROSCOPICAL    PRAXIS. 

and  focussed;  in  Fig.  5  as  it  appears  on  each  side  of  the 
proper  focus;  in  Fig.  6  the  adjustment  is  correct,  but 
the  focus  is  slightly  altered  one  way  or  the  other,  the 
change  being  as  little  as  possible.  In  Fig.  7  the  objec- 
tive is  incorrectly  adjusted,  but  properly  focussed,  while 
Fig.  8  is  the  same  as  seen  beyond  the  focus,  and  Fig.  9 
the  same  when  too  near  the  objective,  or  within  the  fo- 
cus. 

The  late  Dr.  Allen  Y.  Moore,  writing  about  cover- 
correction  says:  Every  objective  has  a  certain  color 
with  which  it  shows  best,  and  there  is  probably  no  ob- 
ject better  adapted  to  the  purpose  of  determining  this 
color  than  the  Podura  scale.  By  examining  the  scale 
with  a  first-class  one-fourth  or  higher  power  of  medium 
or  of  wide  aperture,  it  will  be  seen  that  the  "exclama- 
tion marks"  are  more  or  less  colored.  Pay  no  atten- 
tion to  this  at  first,  but  carefully  turn  the  collar  back 
and  forth  until  the  marks  appear  sharpest  and  smallest. 
That  will  be  the  point  of  best  correction,  and  now  the 
color  of  the  markings  should  be  noticed.  Having  care- 
fully determined  the  exact  tint  of  best  correction,  throw 
the  objective  a  little  out  of  proper  adjustment  by  turn- 
ing the  collar  toward  open  point,  or  zero.  This  over- 
corrects  it  and  at  the  same  time  a  change  in  the  color 
is  noticeable.  The  markings  seem  to  expand,  becom- 
ing hazy.  Now  turn  the  collar  towards  closed  until 
the  point  of  best  correction  is  passed;  here  the  same 
thing  is  seen  in  regard  to  expansion  and  haziness,  but  a 
different  tint  seems  to  make  its  appearance.  By  at- 
tending very  closely  to  this  color  (which  is  the  secon- 
dary spectrum),  the  proper  correction  can  easily  be 
made.  A  certain  one-fifteenth  inch  objective  when 
correctly  adjusted,  shows  the  markings  of  a  brilliant 
ruby-red  (and  most  of  the  finest  objectives  which  I 


MICROSCOPICAL    PRAXIS.  99 

have  seen  show  best  with  this  color);  by  turning  the 
collar  toward  zero  they  become  greenish,  while,  if 
turned  toward  closed,  they  become  pink.  Hence,  at  the 
first  trial  of  any  such  object,  should  it  appear  green  the 
collar  must  be  turned  toward  closed  until  the  ruby  tint 
appears,  and  if  too  pale  a  red  or  pink,  the  collar  should 
be  turned  toward  zero.  By  a  little  practice  the  micros- 
copist  can  tell  at  a  glance  which  way  to  turn  the  collar. 

When  using  this  plan  the  reader  must  remember  that 
all  objectives  do  not  correct  in  the  same  color.  What 
the  special  tint  may  be,  the  microscopist  must  ascertain 
by  noting  the  color  of  the  markings  on  the  scales  when 
the  objective  is  at  the  point  of  best  correction;  he  may 
thereafter  correct  the  lens  by  means  of  its  color  (its  sec- 
ondary spectrum)  for  other  thicknesses  of  the  cover. 
To  do  this  however,  demands  an  eye  very  sensitive  to 
color  and  to  slight  changes  in  tint. 

Mr.  Edward  Bausch,  in  his  little  book  on  "The  Ma- 
nipulation of  the  Microscope,"  prefers  the  diatom  called 
Pleurosigma  angiilatum  as  an  object  over  which  to  make 
cover-corre-ction.  He  recommends  that  the  objective 
be  placed  at  the  open  point  as  usual,  and  focussed  on 
the  diatom.  If  no  lines  are  to  be  seen,  turn  the  collar, 
and  focus  above  and  below  the  plane  of  the  diatom  so 
that  this  shall  be  indistinct,  and  look  for  the  lines. 
Possibly,  after  a  little,  they  will  begin  to  appear  faintly; 
if  not,  continue  to  turn  the  collar  toward  the  closed 
point.  The  lines  must  now  soon  begin  to  make  their 
appearance,  and  when  they  do  so,  they  will  probably 
seem  to  be  above  the  plane  of  the  diatom.  This  shows 
that  the  objective  is  approaching  its  correction  for  the 
cover.  Keep  the  lines  in  focus  while  the  collar  is  being 
gradually  turned,  until  they  and  the  outlines  of  the  dia- 
tom lie  in  one  plane,  when  the  objective  will  be  cor- 


100  MICROSCOPICAL    PRAXIS. 

rected  for  that  cover.  Record  the  number,  and  since 
at  the  beginning  the  adjustment  was  at  the  open  point, 
now  close  it,  and  again  adjust  by  turning  the  collar  the 
other  way.  The  graduations  should  be  at  the  same  de- 
gree in  both  cases,  or  at  least  within  two  degrees  of 
each  other.  If  there  is  a  greater  discrepancy,  Mr. 
Bausch  states  that  it  is  due  to  a  want  of  the  faculty  of 
perception;  the  microscopist's  eye  not  yet  having 
been  sufficiently  trained.  The  reader  must  remember 
however,  that  the  proper  objective  is  to  be  used  with 
the  Pleurosigma,  or  with  any  other  test-object.  With 
this  special  diatom  a  good  one-fourth  or  one-fifth  inch 
or  higher-power  lens  will  be  needed.  To  attempt  to 
resolve  the  lines  with  a  one-inch  objective  would  be 
absurd. 

The  striae  are  more  difficult  to  see  if  the  diatoms  are 
mounted  in  Canada  balsam.  When  dry  under  the 
cover,  they  may  appear  almost  conspicuous,  and  their 
markings  be  easily  observed  with  the  appropriate  ob- 
jective, while  if  the  same  specimens  be  mounted  in  bal- 
sam, they  seem  to  fade  away,  and  must  be  carefully 
sought  for  with  a  low-power  objective  before  a  high- 
power  can  be  used  for  the  resolving  of  the  lines,  or  for 
experimenting  with  the  adjustment-collar.  Before  I 
had  used  the  microscope  much,  and  when  I  was  grop- 
ing about  alone  and  without  help  except  my  own  awk- 
ward failures,  I  received  as  a  present  a  slide  of  Pleuro- 
sigma  angulatum  with  a  cracked  cover-glass.  The  dia- 
toms were  exquisitely  clean  and  beautiful,  but  I  thought 
to  improve  the  appearance  of  the  slide  by  applying  a 
new  cover,  which  I  did  with  Canada  balsam,  the  mount 
being  originally  a  dry  one.  When  I  looked  for  those 
diatoms  with  their  sides  curved  into  the  line  of  beauty, 
they  had  disappeared,  and  in  my  ignorance  I  threw 


MICROSCOPICAL    PRAXIS.  IOI 

away  the  slide,  thinking  that  the  diatoms  had  fallen  off 
and  been  lost.  I  have  often  wished  for  it  since,  for  I 
now  know  that  it  was  a  valuable  thing. 

An  objective  may  be  adjusted  for  the  cover  by  means 
of  the  draw-tube.  The  effect  of  lengthening  or  of 
shortening  the  tube  is  not  so  noticeable  with  low 
powers  as  with  high,  yet  the  same  result  may  be  at- 
tained with  it  as  with  the  adjustment-collar,  but  to  use 
this  method  the  body-tube  must  be  divided.  Shorten- 
ing the  draw-tube  in  this  case  has  the  same  effect  as 
closing  the  lens-systems,  the  objective  then  being  cor- 
rected for  thicker  covers,  while  by  lengthening  the 
tube  it  is  corrected  for  thinner  glass,  or  the  systems  are 
in  effect  opened.  Mr.  Edward  Pennock  has  called  at- 
tention to  this  fact,  stating  that  objectives  of  high- 
power  (%  inch  and  upward),  may  thus  be  used  with  a 
different  length  of  tube  from  that  for  which  they  are 
corrected,  by  using  a  thicker  or  a  thinner  cover-glass. 
For  example,  to  use  an  objective  on  a  long  tube  when 
it  has  been  corrected  i^  fl  snjQJ^"hpj  ng^a  thirmpr 
cqy^r-gTass.  But  to  use  an  objective  on  a  sliort  LUDe  when 
it  has  been  corrected  for  a  long  tube,  use  a  thicker 
cover-glass.  To  use  .with  a  thinner  cover  an  objective 
corrected  for  a  certain  thickness  of  cover-glass, 
lengthen  the  tube  of  the  microscope.  To  use  with  a 
thicker  cover  an  objective  that  has  been  corrected  for  a 
certain  thickness  of  cover-glass,  shorten  the  tube  of  the 
microscope. 

All  non-adjustable  objectives  are  corrected  by  their 
manufacturers  for  a  certain  definite  length  of  body- 
tube,  and  any  change  in  that  length  results  in  an 
injurious  change  in  the  adjustments  and  consequently 
in  the  perfect  action  of  the  lens.  What  that 
special  length  might  be  microscopists  have  had 


102 


MICROSCOPICAL    PRAXIS. 


no  means  of  knowing  until  recently,  when  Prof.  S.  H. 
Gage  of  Cornell  University,  placed 
them  under  renewed  obligations  by 
ascertaining  and  publishing  the  vari- 
ous lengths  employed  by  the  promi- 
nent opticians  of  the  world.  To 
learn  these  Prof.  Gage  sent  the  dia- 
gram, Fig.  10,  to  the  opticians,  ask- 
ing them  to  give  him  the  length  for 
which  they  correct  their  objectives, 
and  to  indicate  the  points  between 
which  the  measurements  are  made. 
Some  measure  from  the  top  of  the 
eye-piece,  to  the  lower  end  of  the 
body,  which  has  been  suggested  as 
a  standard,  since  it  is  readily  ascer- 
tained by  anyone,  and  having  the 
length  once  determined,  says  Prof. 
Gage,  it  would  not  need  to  be 
changed  when  an  objective  of  dif- 
ferent length  of  setting  was  used. 
Others  measure  from  the  top  of  the 
eye-piece  to  the  front  of  the  front 
lens  of  the  objective.  The  follow- 
ing is  the  table  of  lengths  as  given  by  Prof.  Gage, 
with  references  to  the  diagram  so  that  the  points  and 
the  distances  may  be  seen  at  a  glance. 


Fig.  10.    Tube-length 

used  by  various 

opticians. 


MICROSCOPICAL    PRAXIS.  103 

Pts.  included  in 
Tube-lengths. 

See  Diagram  Tube-length  in  Millimetres. 

Fig.  10. 

f  Grunow,  New  York 203  mm. 

,  J  Nachet  et  Fils,  Paris, 146  or  200  mm. 

1  Powell  and  Lealand,  London.   .   .   254  mm. 

[_C.  Reichert,  Vienna 160  to  r8o  mm. 

f  Bausch  &  Lomb  Op.  Co. .Rochester, 21 6  mm. 
Bezu,  Hausser  et  Cie.,  Paris  .   .   .   220  mm. 

11  J  Klonne  und  Miiller,  Berlin  ....    160-180  or  254  mm. 

1  1  W.  &  H.  Siebert,  Wetzlar  ....   190  mm. 

|  Swift  &  Son,  London  • 228*4  mm. 

[_C.  Zeiss,  Jena 160  or  250  mm. 

a-g Gundlach  Optical  Co.,  Rochester,  254  mm. 

c-d Ross  &  Co.,  London, 254  mm. 

c-e R.  &.  J.  Beck,  London 254  mm. 

c-g H.  R.  Spencer  &  Co., Geneva, N.Y*., 254  mm. 

c-e E.  Leitz,  Wetzlar  ) 125-180  mm. 

Oil  immersions       ) 160  mm. 


Some  objectives  have  the  collar  so  marked  and  ar- 
ranged by  the  optician  that  by  it  alone^without  examin- 
ing the  image,  the  adjustment  for  cover  may  be  made, 
provided  the  thickness  of  the  cover-glass  be  known. 
If.  the  cover  is,  for  instance,  0.15  mm.  thick,  the  collar 
should  be  placed  at  15;  if  the  cover  is  0.25  mm.,  set  the 
collar  at  25.  This  .method  has  been  adopted  in  this 
country  by  Messrs.  Bausch  &  Lomb,  and  by  Carl 
Reichert  and  others  in  Europe. 


104  MICROSCOPICAL    PRAXIS. 

Chromatic  Aberration. 

With  the  exception  of  Zeiss's  apochromatic  objectives 
none  is  perfectly  corrected  for  chromatic  and  for 
spherical  aberration.  There  is  in  all  a  remnant  of  color 
and  a  remnant  of  spherical  trouble  which  it  is  impossi- 
ble to  dispose  of,  except  by  using  Prof.  Abbe's  apo- 
chromatic devices.  In  what  is  called  the  chromatic  or 
the  color-aberration  of  the  objective,  the  different 
colored  rays  are  not  all  brought  to  the  same  focus,  and 
in  his  efforts  to  remedy  these  defects  the  optician  does 
either  too  much  or  too  little.  When  the  violet  rays  are 
focussed  before  the  red,  the  objectives  is  said  to  be 
under-corrected;- when  the  red  light  is  focussed  before 
the  violet,  the  lens  is  over  corrected. 

Mr.  Edward  Bausch,  the  optician,  in  his  little  work 
on  "The  Manipulation  of  the  Microscope,"  says  that 
the  amount  of  color  depends  somewhat  upon  the  power 
of  the  eye-piece,  becoming  more  conspicuous  as  the 
power  is  increased,  and  that  the  color  outside  of  the 
secondary  spectrum  is  not  always  prejudicial.  The 
microscopist  must  depend  upon  the  optician  for  the  ex- 
cellence of 'these  corrections,  and  the  optician  will 
serve  him  well,  but  it  is  always  interesting,  and  often 
useful,  to  be  able  to  test  the  perfection  of  the  maker's 
results.  So  far  as  chromatic  aberration  is  concerned 
this  may  be  done  in  several  ways. 

If  an  organic  object  is  to  be  employed  as  a  test,  the 
most  useful  are  the  Podura  scale  and  a  diatom.  Mr. 
Edward  Pennock  has  published  a  convenient  table  of 
color  corrections  which  I  quote  as  follows: 


MICROSCOPICAL    PRAXIS. 


Table  of  Color  Corrections. 

Within  the  focus.        Without  the  focus. 


Under  correction,    .     . 

Slightly  under,  but  a^| 
large  number  of  the  I 
finest  lenses  have  this  f 
color, J 

Nearly  colorless,  shows 
the  secondary  spec- 
trum   

Over  correction,      .     . 


Brick  red 


Claret 


Lilac 


Blue 


Greenish  blue. 


Light  green. 


Paler  green. 


Yellow. 


To  make  the  appearances  conspicuous  the  mirror 
should  be  swung  to  one  side  and  the  colored  fringes 
(the  complimentary  colors  of  the  secondary  spectrum), 
at  the  margins  of  the  object  be  noticed,  and  the  object, 
should,  as  in  Mr.  Pennock's  table,  be  examined  both 
within  and  without  the  focus,  the  first  mentioned 
method  by  central  illumination  being  the  best  for  the 
learned  optician  whose  eye  has  been  acutely  trained  to 
observe  slight  chromatic  variations.  When  the  object 
is  viewed  under  oblique  light  from  the  mirror  swung 
toward  the  right-hand  side,  the  left-hand  margin  of  the 
object  will  be  bordered  with  violet,  and  the  right-hand 
side  by  yellow  if  the  objective  is  over  corrected;  when 
under-corrected  the  left-hand  margin  will  be  yellow  and 
the  opposite  edges  blue  or  violet,  a  low-power  eye- 
piece being  used. 

When  an  inorganic  substance  is  used  as  the  test,  al- 
most any  small  object  orbit  of  dirt  will  answer  the  pur- 
pose, and  in  every  instance  that  object  should  be  as 
nearly  as  possible  in  the  centre  of  the  field. 

Professors  Naegeli  and  Schwendener  however, 
recommend  as  the  simplest  and  most  trustworthy 
mode  of  testing  the  chromatic  aberration,  that  one-half 


IO6  MICROSCOPICAL    PRAXIS. 

the  surface  of  either  the  front  or  of  the  back  lens  of  the 
objective  be  covered  with  tin-foil  or  with  black  paper, 
so  that  only  one-half  shall  remain  optically  effective.  If 
a  minute  aperture  in  a  blackened  plate  then  be  ex- 
amined, it  will  appear  colorless  if  the  objective  is  per- 
fectly achromatic,  as  no  ordinary  objective  can  be;  or 
the  margins  will  be  colored  as  described  in  the  foregoing 
experiments  with  oblique  light.  It  is  better,  however, 
not  to  meddle  much  with  the  glass  surfaces  of  the  ob- 
jective. And  this  method  has  also  some  other  objec- 
tionable features. 

With  high-powers  these  methods  are  sufficient,  but 
with  low-powers  the  opticians  recommend  the  use  of  a 
minute  globule  of  mercury  intensely  illuminated  by  re- 
flected light.  This  is  the  artificial  star  so  often  referred 
to  in  microscopical  literature.  It  is  made  by  beating 
with  a  broad  blade  a  globule  of  mercury  until  it  is  as 
fine  as  dust.  These  minute  globules  are  then  used  as 
reflecting  surfaces,  and  are  preferably  mounted  on  a 
slip  of  black  glass,  as  recommended  by  Mr.  W.'H.  Wen- 
ham. 

If  the  objective  is  well  corrected  the  colored  fringes 
will  be  pale  green  when  the  globule  is  without  the  fo- 
cus, and  pale  violet  when  it  is  within.  If  the  lens  is 
under-corrected,  the  green  will  become  bluish  or  violet, 
and  when  within  the  focus  the  color  may  be  bright  red. 
If  over-corrected,  the  colors  are  yellow  and  blue,  when 
without  and  within  the  focus  respectively. 


MICROSCOPICAL    PRAXIS.  107 

Spherical  Aberration. 

Of  the  two  troubles,  chromatic  and  spherical  aberra- 
tion, the  latter  is  the  more  important  to  be  entirely 
overcome;  yet,  except  in  the  Zeiss  apochromatics  there 
is,  even  in  the  best  objectives,  some  of  it  left,  to  which 
the  same  expressions  of  under  and  over-correction  are 
applied.  When  the  central  rays  meet  before  the  per- 
ipheral ones,  the  objective  is  said  to  be  over-corrected; 
when  the  peripheral  rays  meet  first,  the  lens  is  under- 
corrected. 

In  the  combination  pocket-lens  the  spherical  as  well 
as  the  chromatic  aberration  is  partly  corrected  by  a  dia- 
phragm which  obstructs  the  passage  of  the  peripheral 
rays.  For  this  reason  the  object  appears  more  dis- 
tinct, but  the  size  of  the  field  and  the  amount  of  light 
are  much  reduced.  In  some  objectives  this  plan  is 
adopted  when  the  corrections  are  not  made  by  the  com- 
ponent lenses  themselves,  with  much  the  same  effect  as 
in  the  case  of  the  combination  pocket-lens. 

As  a  test  for  spherical  aberration  the  optician  makes 
use  of  the  artificial  star.  This  is  done  by  examining  it 
within  and  without  the  focus,  and  studying  the  coma, 
or  the  expansions  of  the  margins.  If  the  expansion  is 
greater  when  the  distance  between  the  objective  and 
the  star  is  increased,  the  objective  is  under-corrected; 
if  the  expansion  is  greater  when  the  reverse  obtains, 
the  objective  is  over-corrected. 

A  more  accessible  test  for  the  general  reader  is  that 
made  by  coating  a  slip  of  glass  with  a  thick  layer  of 
India  ink,  which  will  crack  into  minute  fissures  when 
dry,  or  in  which  a  fine  needle-point  may  make  delicate 
marks  for  the  transmission  of  light.  In  using  this  and 
all  other  tests  for  the  same  purpose,  with  low-powers,  the 
diaphragm  must  be  removed  and  the  mirror  be  brought 


I08  MICROSCOPICAL    PRAXIS. 

nearer,  so  that  the  cone  of  rays  shall  fill  the  whole 
aperture  of  the  objective;  and  with  high-powers  a  sub- 
stage  condenser  should  be  used  for  the  same  purpose. 
The  objective  is  sharply  focussed  on  the  edges  of  the 
fissures,  and  if  these  appear  with  a  blue  fog  the  aberra- 
tion is  not  sufficiently  corrected.  They  should  be  seen 
without  a  halo,  clearly  and  distinctly.  If  the  foggy  ap- 
pearance increases  within  the  focus,  the  objective  is 
over-corrected;  when  it  increases  without  the  focus  the 
lens  is  under-corrected. 

For  testing  both  forms  of  aberration,  Prof.  Abbe  has 
devised  a  test-plate  consisting  of  a  series  of  cover- 
glasses  ranging  in  thickness  from  0.09  mm.  to  0.24  mm., 
their  lower  surfaces  being  coated  with  a  silver  film 
through  which  are  ruled  groups  of  lines  varying  in  dis- 
tance from  3-J-g-  to  -j-g^g-  inch.  These  covers  are  ce- 
mented to  the  slip  by  Canada  balsam.  To  examine  an 
objective  of  large  aperture,  the  inventor  says  in  his  in- 
structions accompanying  the  plate,  the  disks  must  be 
focussed  in  succession,  observing  in  each  case  the 
quality  of  the  image  in  the  centre  of  the  field,  and  the 
variation  produced  by  using  alternately  central  and 
very  oblique  illumination.  When  the  objective  is  per- 
fectly corrected  for  spherical  aberration  for  the  partic- 
ular thickness  of  cover-glass  under  examination,  the 
outlines  of  the  markings  in  the  centre  of  the  field  will 
be  sharp  by  oblique  illumination,  and  without  any  neb- 
ulous doubling  or  indistinctness  of  the  minute  irregul- 
arities of  the  edges.  If,  after  exactly  adjusting  the  ob- 
jective for  oblique  light,  central  illumination  is  used,  no 
alternation  of  the  focus  should  be  necessary  to  show 
the  outlines  with  equal  sharpness. 

If  an  objective  fulfills  these  conditions  with  any  one 
of  the  disks  it  is  free  from  spherical  aberration  when 


MICROSCOPICAL    PRAXIS.  IOQ 

used  with  cover-glasses  of  that  thickness.  On  the 
other  hand,  if  every  disk  shows  nebulous  doubling  or 
an  indistinct  appearance  of  the  edges  of  the  lines  with 
oblique  illumination,  or  if  the  objective  requires  a  dif- 
ferent focal  adjustment  to  get  equal  sharpness  with 
central  as  with  oblique  light,  then  the  spherical  correc- 
tion of  the  objective  is  more  or  less  imperfect. 

Nebulous  doubling  with  oblique  illumination  indi- 
dicates  over-correction  of  the  marginal  zone,  indistinct- 
ness of  the  edges  without  marked  nebulosity  indicates 
under-correction. 

The  test  for  chromatic  aberration  is  based  on  the 
character  of  the  color-bands  which  are  visible  with 
oblique  illumination.  With  good  correction  the  edges 
of  the  lines  in  the  centre  of  the  field  should  show  only 
narrow  color-bands  in  the  complimentary  colors  of  the 
secondary  spectrum,  namely  on  one  side  yellow-green 
to  apple-green,  and  on  the  other  violet  to  rose.  The 
more  perfect  the  correction  of  the  spherical  aberration 
the  clearer  this  color-band  appears. 

For  the  examination  of  objectives  of  smaller  aper- 
ture (less  than  40°  or  50°),  we  may  obtain  all  the  necessary 
data  for  the  estimation  of  the  spherical  and  the  chro- 
matic corrections  by  placing  the  concave  mirror  so  far 
laterally  that  its  edge  is  nearly  in  the  line  of  the  optic 
axis,  the  incident  cone  of  rays  then  filling  only  one- 
half  of  the  aperture  of  the  objective,  by  which  means 
the  sharpness  of  the  outlines  and  the  character  of  the 
color-bands  can  be  easily  estimated.  Differences  in  the 
thickness  of  the  cover-glass  within  the  ordinary  limits 
are  scarcely  noticeable  with  such  objectives. 

It  is  of  fundamental  importance  in  employing  this  test- 
plate  to  have  brilliant  illumination  and  to  use  an  eye- 
piece of  high  power.  With  oblique  illumination  the  light 


110  MICROSCOPICAL    PRAXIS. 

must  always  be  thrown  perpendicularly  to  the  direc- 
tion of  the  lines. 

When,  with  practice,  the  eye  has  learned  to  recog- 
nize the  finer  differences  in  the  quality  in  the  outlines 
of  the  images,  this  method  of  investigation  gives  very 
trustworthy  results.  Differences  in  the  thickness  of 
cover-glasses  of  o.oi  to  0.02  mm.  can  be  recognized 
with  objectives  of  two  or  three  mm.  focus. 

Objectives  are  commonly  more  nearly  free  from 
spherical  aberration  in  the  centre  than  at  the  peri- 
phery. It  is  for  this  reason  that  when  a  minute  object 
is  to  be  examined  wit]?  a  high-power,  the  advice  always 
given  is  to  bring  it  to  the  centre  of  the  field.  This 
must  be  specially  remembered  when  measuring  objects 
that  extend  too  near  the  sides  of  the  field,  since  the 
spherical  aberration  of  the  lateral  portions  of  the  lens 
has  the  effect  of  increasing  or  of  dimnishing  the  am- 
plification, and  the  measurement  will  not  be  correct  if 
the  aberration  is  at  all  marked.  This  result  of  spheri- 
cal defect  is  usually  very  noticeable  in  what  is  called 
flatness  of  field. 


Flatness  of  Field. 

Occasionally  the  field,  instead  of  appearing  as  a  flat 
or  level  plane,  seems  to  be  like  the  inside  or  the  outside 
of  a  shallow  cup,  and  although  this  is  one  result  of 


MICROSCOPICAL    PRAXIS.  Ill 

spherical  aberration,  the  latter  alone  cannot  be  justly 
blamed.  In  some  objectives  the  curvature  is  so  great 
that  the  focus  must  be  changed  to  a  very  perceptible 
extent  before  marginal  objects  can  be  clearly  seen,  after 
which  the  central  portions  become  indistinct.  This 
curvature  should  be  as  slight  as  possible,  although, 
since  it  cannot  be  entirely  eliminated,  except  by  the 
use  of  apochromatic  glass  and  of  special  eye-pieces  it 
must  be  endured. 

It  may,  as  stated  by  Mr.  Edward  Bausch  be  due  to  a 
defective  eye-piece.  This  may  be  determined  by  observ- 
ing whether  it  shows  equally  with  different  objectives, 
and  by  using  different  eye-pieces,  but  of  the  same  fo- 
cal length,  with  the  same  objectives.  And  Mr.  Bausch 
gives  good  advice  when  he  says  that  in  two  objectives, 
if  the  predominant  feature  of  one  be  resolving  power, 
and  of  the  other  flatness  of  field,  select  the  former. 

A  simple  method  of  testing  the  flatness  of  the  field 
is  to  strew  starch-grains  over  a  slide,  and  to  observe 
whether  the  centre  and  the  periphery  of  the  field  of 
view  are  in  exactly  the  same  focus  at  the  sime  time. 
If  the  centre  is  focussed  and  the  margin  appears  hazy 
and  indistinct,  as  it  probably  will  appear  even  with  the 
best  objectives,  the  field  is  not  perfectly  flat.  Similarly 
minute  substances,  as  minute  bacteria,  strewn  over  the 
lower  surface  of  the  cover-glass  will  tell  the  same  story 
with  high-power  objectives.  Perfect  flatness  of  field, 
even  with  the  best  of  lenses,  does  not  exist. 

Professors  Naegeli  and  Schwendener  give  the  follow- 
ing method  for  testing  this  feature  in  objectives.  A 
cover-glass  with  a  straight  edge  is  placed  upon  the  dia- 
phragm of  the  eye-piece  (the  eye  lens  having  been  re- 
moved), so  that  the  edge  shall  appear  in  the  circular 
opening  of  the  diaphragm  as  a  chord  of  arc.  The  real 


112  .         MICROSCOPICAL    PRAXIS. 

image  of  another  straight  line,  which  is  viewed  as  an 
object,  is  made  to  coincide  with  this  chord  precisely  as 
the  adjustment  of  a  particular  division-line  to  the  mar- 
gin of  an  object  is  effected  in  micrometric  measure- 
ments. If  a  complete  coincidence  takes  place — 
whether  or  not  they  appear  straight  or  curved  in  the 
virtual  image — then  the  real  image  is  free  from  distor- 
tion; in  all  other  cases  it  is  distorted. 


Angular  Aperture. 

The  older  opticians  defined  angular  aperture  to  be 
the  angle  at  the  apex  of  a  tri-angle  whose  base  is  repre- 
sented by  the  width  of  the  front  lens  of  the  objective, 
its  two  sides  being  the  most  oblique  rays  of. light  which 
can  pass  through  that  objective  from  a  point  in  the 
object  when  focussed,  that  point  being  at  the  apex  of 
the  triangle.  But  as  the  result  of  the  investigations 
made  by  Professor  Abbe  of  Jena,  objectives  are  now 
rated  according  to  what  he  has  named  the  Numerical 
Aperture  (N.  A.).  This  is  one-half  of  the  sine  of  the 
angular  aperture  multiplied  by  the  refractive  index  of 
the  medium  in  which  the  objective  is  used,  which  may 
be  air,  as  it  must  be  for  a  dry  objective,  water,  glycer- 
ine or  some  homogeneous-immersion  fluid. 


MICROSCOPICAL    PRAXIS.  113 

The  refractive  index  of  air  is  taken  to  be  i.ooo;  of 
water  it  is  1.333;  °f  glycerine,  1.475;  °f  °^  °f  cedar, 
1.510;  of  Canada  balsam,  1.540;  of  styrax,  1.582;  of 
crown  glass,  1.51  to  1.53. 

The  angular  or  the  numerical  aperture  may  be  ob- 
tained by  the  simple  methods  described  in  another 
place,  or  the  N.  A.  may  be  ascertained  from  the  follow- 
ing table  when  the'  angular  aperture  has  been  learned 
from  the  optician.  The  list  is  taken  from  "The  Journal 
of  the  Royal  Microscopical  Society,"  for  which  it  was 
prepared  by  Mr.  J.  W.  Stephenson,  to  whom  microsco- 
pists  owe  the  use  of  homogeneous-immersion  objec- 
tives, as  he  was  the  first  to  suggest  a  practicable  means 
of  utilizing  the  optical  principle  involved  in  their  con- 
struction and  employment. 

A  glance  at  the  table  will  show  how  to  use  it  to 
ascertain  the  N.  A.  If  a  dry  objective  have  an  angular 
aperture  of  125°  (in  the  column  headed  "Air"),  it  should 
correspond  in  resolving  power  and  in  some  other  good 
qualities,  with  a  wTater-immersion  having  an  angular 
aperture  of  84°,  and  with  a  homogeneous  immersion  of 
71°,  while  its  N.  A.  will  #€"0.89. 


MICROSCOPICAL    PRAXIS. 

APERTURE  TABLE. 


Numerical  i      .  . 
Aperture. 

Water. 

Homogeneous 
Immersion, 

1  -52 

180°  o' 

1.51 

.  . 

.  . 

166°  51' 

1.50 

.  . 

,  . 

161°  23' 

i.49 

157°  12' 

1.48 

. 

153°  39' 

1-47 

. 

150°  32' 

1.46 

.  . 

147°  42' 

1-45 

. 

.  . 

145°  6' 

i.44 

.  . 

.  . 

142°  39' 

i-43 

.  . 

140°  22' 

1.42 

. 

I38°  12' 

1.41 

.  . 

136°  8' 

1.40 

134°  10' 

1-39 

.  . 

132°  16' 

1.38 

.  . 

130°  26' 

1-37 

128°  40' 

1.36 

126°  58' 

•1-35 

.  . 

125°  18' 

. 

.  . 

123°  40' 

1-33 

.  . 

1  80°   0' 

122°   6' 

1.32 

.  . 

165°  56' 

120°  33' 

I-31 

160°  6' 

119°  3' 

1.30 

155°  38' 

"7°  35' 

1.29 

151°  5o' 

116°  8' 

1.28 

. 

148°  42' 

114°  44' 

1.27 

145°  27' 

I  13°  21 

1.26 

142°  39 

in0  59' 

I-25 

140°  3' 

no0  39' 

1.24 

, 

i37°  36' 

109°  20' 

1.23 

135°  i7' 

108°  2' 

1.22 

133°  4' 

106°  45' 

1.  21 

130°  57' 

105°  30' 

1.20 

128°  55' 

104°  15' 

I.I9 

t 

126°  58' 

103°   2 

1.18 

125°  3' 

101°  50' 

I.I7 

123°  13' 

100°  38' 

1.16 

121°  26' 

99°  29' 

MICROSCOPICAL    PRAXIS. 


APERTURE  TABLE— Continued. 


Numerical 
Aperture. 

Air. 

Water. 

Homogeneous 
Immersion. 

•IS 

119°  41' 

98°  20' 

•  14 

.  . 

118°  o' 

97°  ii' 

•*3 

.  . 

116°  20' 

96°   2' 

.12 

.  . 

114°  44' 

94°  55' 

.11 

H3°  9' 

93°  47' 

.10 

ni°  36' 

92°  43' 

.09 

110°  5' 

9i°  38' 

.08 

. 

108°  36' 

90°  34' 

.07 

.  . 

107°  8' 

89°  30' 

.06 

.  . 

105°  42' 

88°  27' 

•°5 

.  . 

O    s  I 

104  16 

87°  24' 

.04 

102°  53' 

86°  21' 

•03 

101°  30' 

85°  19' 

.02 

100°  10' 

84°  1  8' 

.01 

98°  50' 

83°  17' 

.00 

1  80°'  o' 

97°  3i' 

82°  17' 

0.99 

163°  48' 

96°  12' 

81°  17' 

0.98 

157°   2' 

94°  56' 

80°  17' 

0.97 

'51°  52' 

93°  4o' 

79°  18' 

0.96 

147°  29' 

92°  24' 

78°  20' 

°-95 

143°  36' 

91°  10' 

77°  22' 

0.94 

140°  6' 

89°  56 

76°  24' 

°-93 

136°  52' 

88°  44' 

75°  27' 

0.92 

133°  5'' 

87°  32' 

74°  30'- 

0.91 

131°  o' 

86°  20' 

73  33' 

0.90 

128°  19' 

85°  10' 

72  36' 

0.89 

125°  45' 

84°  o' 

7i°  40' 

0.88 

123°  17' 

82°  51' 

70°  44' 

0.87 

120°  55' 

81°  42' 

69°  49' 

0.86 

118°  38' 

80°  34' 

68°  54' 

0.85 

116°  25' 

79°  37' 

68°  o' 

0.84 

114°  17' 

78°  20' 

67°  6' 

0.83 

112°  I2' 

77°  i4 

66°  12' 

0.82 

110°  10' 

76°  8' 

65°  18' 

0.81 

108°  10' 

75°  3' 

64°  24^ 

0.80 

106°  16' 

73°  58' 

63°  3i' 

0.79 

104°  22' 

72°  53' 

62°  38' 

n6 


MICROSCOPICAL    PRAXIS. 


APERTURE  TABLE— Continued. 


Numerical 
Aperture. 

Air. 

Water. 

Homogeneous 
Immersion  . 

0.78 

102°    3l' 

7i°  49' 

61°  45' 

0.77 

100°  42' 

70°  45' 

60°  52' 

0.76 

98°  56' 

69°  42' 

60°     o' 

0-75 

97°   n' 

68°  40' 

59°     8' 

0.74 

95°  28' 

67°  37' 

58°   16' 

0-73 

93°  46' 

66°  34' 

57°  24' 

0.72 

92°     6' 

65°  32' 

56°  32' 

0.71 

90°  28' 

64°  32' 

55°  4i' 

0.70 

88°  51' 

63°  3i' 

54°  50' 

0.69 

87°   16' 

62°  30' 

53°  59' 

0.68 

85°  41' 

61°  30' 

53°     9 

0.67 

84°     8' 

60°  30' 

52°   18' 

0.66 

82°  36' 

59°  3o' 

51°   28' 

0.65 

81°     6' 

58°  30' 

50°  38' 

0.64 

79°  36' 

57°  3i' 

49°  48' 

0.63 

78°     6' 

56°  32' 

48°  58' 

0.62 

76°  38' 

55°  34' 

48°     9' 

0.6  1 

75°  10' 

54°  36' 

47°   19' 

0.60 

73°  44 

53°  38' 

46°  30' 

0-59 

72°   18' 

52°  4o| 

45     4o 

0.58 

7°:  54; 

5!°    42' 

o            / 

O               / 

44     51 

O                / 

0-57 

09   30 

5o°  45 

44       2 

0.^6 

68°     6' 

49°  48' 

43°   14 

°-55 

66°  44' 

49°  Si' 

42°   25' 

o.54 

65°  22' 

47°  54 

O                f 

4i     37, 

0-53 

64°     0^ 

46°  58' 

40°  48' 

0.52 

62°  40' 

46°        2 

40°     o' 

Q-51 

61°  20' 

45°     6' 

39°o   12' 

0.50 

60°     o' 

44°   10' 

38°  24; 

0.48 

57°  22' 

42°   18' 

36°  49' 

0.46 

54    47' 

40°   28' 

351  is; 

0-45 

53°  3o' 

39°  33' 

34°  27 

0.44 

52°   13' 

38°  38' 

33     4° 

0.42 

O                 f 

49     4° 

36°  49' 

32°     5 

0.40 

47°     9 

35°     o' 

3°°  31' 

0.38 

O               f 

44     4° 

33°  12' 

28°  57' 

0.36 

42°    12' 

31°  24' 

O                ? 

27    24 

MICROSCOPICAL    PRAXIS. 


117 


APERTURE 


Numerical 
Aperture. 

Air. 

Water. 

Homogeneous 
Immersion. 

°-35 

40°   58' 

30°    30' 

26°    38' 

0-34 

39°  44' 

29°  37' 

25°    51' 

0.32 

37°   20' 

27°    5l' 

24°   1  8' 

0.30 

34°  56' 

26°      4' 

22°    46' 

0.28 

32°  32' 

24°   18' 

21°    I4' 

0.26 

30°   10' 

22°  33' 

19°    42' 

0.25 

28°  58' 

21°    40' 

18°  56' 

0.24 

27°  46' 

20°    48' 

18°   10' 

0.22 

25°  26' 

I9°        2' 

16°  38' 

0.20 

23°     4' 

17°  18' 

i5°     7' 

0.18 

20°    44' 

15°  34' 

i3°  36' 

o.i  6 

18°   24' 

13°  5o' 

12°     5' 

0.15 

17°   14' 

12°  58' 

n°  19' 

0.14 

16°     5'             12°     6' 

10°  34' 

0.12 

13°  47' 

9°     4' 

0.10 

12°  29'                8°  38' 

7°  34' 

0.08 

O                / 

9     ii 

'    6°  54' 

6°     3' 

0.06 

6°  53'               5°   10' 

4°  32' 

0.05 

5°  44'               4°   18' 

3°  46' 

To  Measure  Angular  Aperture. 

The  optician  usually  engraves  the  angle  of  aperture 
on  the  mounting  of  the  best  objectives,  and  with  the 
immersion  lenses  this  will  be  the  numerical  aperture  (N. 
A.).  For  the  correctness  of  the  measurement  the  mi- 
croscopist  may  trust  the  optician,  yet  he  may  readily 


Il8  MICROSCOPICAL    PRAXIS. 

measure  the  angle  for  his  own  satisfaction,  by  methods, 
which,  although  perhaps  not  as  accurate  as  those  of  the 
manufacturer,  are  sufficiently  so  for  the  needs  of  the 
amateur. 

If  the  microscope-stand  have  a  graduated,  revolving 
platform  beneath  the  pillars,  nothing  else  will  be  needed 
except  a  candle  which  should  be  placed  at  one  end  of 
the  table,  and  so  that  the  flame  shall  be  about  on  a 
level  with  the  objective  to  be  measured  when  the  mi- 
croscope is  in  a  horizontal  position.  Remove  the  mir- 
ror and  all  sub-stage  apparatus,  place  the  ocular  in  po- 
sition, and  when  the  field  has  been  evenly  lighted,  ro- 
tate the  horizontal  instrument  to  one  side  until  one-half 
the  field  is  dark,  the  line  of  demarkation  between  the 
light  and  the  dark  halves  being  made  as  nearly  central 
and  perpendicular  as  possible.  The  graduation  on  the 
base  of  the  stand  will  be  one-half  of  the  angle  of  aper- 
ture (not  the  numerical  aperture),  and  may  be  verified 
by  rotating  the  microscope  to  the  other  side,  and  again 
bisecting  the  field  with  the  shadow,  when  the  reading 
should  give  the  same  degree. 

The  value  of  the  reading  has  been  said  to  be  one- 
half  of  the  angle  of  aperture,  but  this  is  contingent  upon 
the  arrangement  of  the  graduations.  If  the  instrument 
was  at  zero  at  the  beginning  of  the  experiment,  this 
holds  good;  but  if  the  platform  is  so  graduated,  as  it  is 
with  some  stands,  that  the  starting  point  cannot  con- 
veniently be  at  -zero,  then  the  instrument  must  be  ro- 
tated in  both  directions,  and  the  difference  therefore  of 
the  graduations  will  be  the  angular  aperture.  On  my 
own  stand  the  graduations  begin  at  zero  and  advance 
both  ways,  so  that  a  low-power  objective  just  measured, 
gives  5°  when  rotated  to  one  side,  the  aperture  conse- 
quently being  ten  degrees.  For  the  sake  of  verifying 


MICROSCOPICAL    PRAXIS.  1 19 

this  result,  the  base  was  so  turned  that  the  rotation  be- 
gan at  50°.  When  one-half  the  field  was  bisected,  the 
graduation  marked  56°;  when  rotated  in  the  other  direc- 
tion, the  instrument  stood  at  46°,  the  difference  giving 
the  same  angle  as  in  the  former  experiment. 

If  the  stand  have  no  graduated  platform,  the  entire 
instrument,  in  the  same  conditions  and  in  the  same  po- 
sition, may  be  rotated.  On  a  sheet  of  paper  draw  a  cir- 
cle whose  diameter  is  equal  to  the  greatest  length  of 
the  foot  of  the  stand.  Rotate  the  microscope  within 
this  circle  until  the  field  is  exactly  bisected,  and  mark 
the  paper  against  one  side  of  the  foot;  rotate  it  in  the 
opposite  direction,  and  when  the  other  side  of  the  field 
is  again  bisected,  make  against  the  same  side  of  the 
foot  another  mark  on  the  paper.  Extend  these  two 
lines  until  they  meet,  and  measure  the  included  angle 
with  the  common  protractor.  This  will  give  the  angu- 
lar aperture  of  that  objective  in  air. 

The  accuracy  of  all  these  measurments  will  depend 
entirely  upon  the  experimenter's  carefulness  in  bisect- 
ing the  field  of  the  objective,  and  in  measuring  the  angle 
with  the  protractor.  Slight  errors  at  the  beginning 
will  have  a  painfully  conspicuous  appearance  at  the 
end. 

Should  the  reader  unfortunately  have  a  stand  with- 
out a  joint  for  inclination,  he  may  still  measure  the 
angles  of  his  dry  lenses  by  a  method  suggested  by  an 
anonymous  writer.  With  the  microscope  placed  in  a 
vertical  position  on  a  table,  preferably  a  table  with  a 
dark-colored  cover,  the  eye-piece,  the  mirror  and  all 
the  sub-stage  apparatus  are  to  be  removed.  Rack 
down  the  body-tube  until  the  objective  passes  as  far  as 
possible  through  the  stage-opening.  Place  two  pieces 
of  white  card  one  on  each  side  of  the  instrument,  and 


120  MICROSCOPICAL    PRAXIS. 

while  looking  down  the  body-tube,  move  them  back 
and  forth  until  both  can  be  just  seen  at  the  opposite 
and  outer  edges  of  the  field  of  the  objective.  Measure 
the  distance  from  the  inside  margin  of  one  card  to  the 
inside  margin  of  the  other,  and  the  distance  of  the 
front  of  the  lens  from  the  table.  Draw  the  first  men- 
tioned distance  as  a  horizontal  base-line,  and  the  latter 
as  a  perpendicular  to  its  centre.  Connect  both  sides 
of  the  base  with  the  top  of  the  vertical  line,  and  the 
angle  at  the  apex,  or  more  correctly  the  two  angles 
formed  with  the  perpendicular  at  the  apex  of  the 
triangle,  will  represent  the  aperture  of  the  objective, 
and  may  be  measured  with  the  protractor. 

The  adjustment-collar  usually  has  an  influence  in 
this  connection.  When  the  objective  is  to  be  measured 
for  angle,  the  adjustment  should  be  at  or  near  closed, 
as  that  is  usually  the  point  of  maximum  aperture. 

For  the  purpose  of  measuring  angular  aperture  the 
optician  and  the  students  of  microscopical  optics 
employ  a  special  apparatus  termed  an  apertometer,  of 
which  there  are  several  forms  in  use  and  with  which 
the  end  desired  may  be  attained  with  great  accuracy. 


The  Abbe  Apertometer. 

The  simplest,  perhaps  the  best  apertometer,  at  least 
the  best-known,  is  that  devised  by  Professor  Abbe,  of 
Jena,  and  made  by  the  accomplished  optician,  Carl 
Zeiss  and  by -his  successors.  It  consists  of  a  heavy, 


MICROSCOPICAL    PRAXIS.  121 

semi-circular  disk  of  glass  graduated  on  its  bevelled 
edge,  and  bearing  two  movable  metal  clips,  one  on 
each  side,  which  mark  the  limits  of  the  aperature  in  a 
way  similar  to  that  already  described  in  the  simple 
methods  for  measuring  aperture  referred  to  on  a  pre- 
ceding page.  A  low-power  objective  accompanies  the 
apparatus.  The  disk  is  placed  on  the  horizontal  stage 
of  the  microscope,  the  objective  to  be  measured  being 
focussed  over  a  central  spot  on  its  surface,  when  the 
metal  clips  are  to  be  moved  on  each  side  essentially 
as  already  described,  and  the  angle  is  read  off  between 
the  clips.  The  apparatus  is  useful  if  the  microscopist  have 
many  objectives  to  measure,  as  a  manufacturing  optician 
or  a  dealer  might  have,  but  to  the  general  reader  it 
would  be  merely  an  optical  luxury,  to  be  possessed  if  he 
be  wealthy,  but  not  to  be  specially  desired  if  he  must  be 
a  little  economical.  The  angular  aperture  may  be 
ascertained  by  any  of  the  simple  methods  already  de- 
tailed, and  the  numerical  aperture  read  off  from  the 
table  given  on  another  page. 

Many  objectives,  perhaps  the  majority  of  even  first- 
class  objectives,  admit  rays  of  light  around  and  from 
near  the  margins  of  the  lens  which  have  no  part  in  the 
formation  of  the  image,  except  in  some  cases  to  produce 
an  imperfect  result  by  their  interference  with  the  rays 
which  enter  into  and  emerge  from  the  more  nearly  cen- 
tral area  of  the  lens.  These  oblique  rays  do  not  reach 
the  same  focus  which  the  more  central  rays  reach,  the 
result  being  a  possible  deterioration  in  the  perfect 
action  of  the  objective.  Yet  when  measuring  their 
lenses  for  angular  aperture,  the  majority  of  opticians 
measure  not  only  the  more  central,  but  also  the  mar- 
ginal rays  which  may  have  a  bad  influence  on  the  ob- 
jective's action.  This  is  not  quite  fair  to  the  purchaser 


122  MICROSCOPICAL    PRAXIS. 

yet  it  is  commonly  done.  It  is  done  by  all  opticians, 
so  far  as  I  have  been  able  to  learn,  except  by  the  late 
Robert  B.  Tolles,  of  Boston,  and  by  Mr.  Herbert  Spen- 
cer of  the  Spencer  and  Smith  Optical  Company,  of 
Buffalo,  N.  Y.  These  two  accomplished  opticians 
measure  only  the  rays  which  are  truly  image-forming. 
This  method  of  ascertaining  the  angular  aperture  of  an 
objective  originated  with  the  late  R.  B.  Tolles,  and  is 
an  eminently  honest  and  praiseworthy  manner  of  deal- 
ing not  only  with  the  objective,  but  with  the  purchaser 
who  puts  his  trust  in  the  manufacturing  optician,  and 
has  no  kind  feelings  toward  that  man  when  he  is  deceived. 
The  majority  of  objectives  when  measured  by  the  image- 
forming  rays  only,  will  be  reduced  in  angle  in  a  way  that 
will  be  painful  to  the  unsuspecting  owner.  The  usual 
method,  the  one  which  has  thus  far  been  been  described 
in  the  preceding  pages,  measures  not  the  true  angular 
aperture,  although  it  is  always  called  by  that  name, 
but  it  measures  the  angle  of  the  field,  quite  another 
thing,  because  the  field  will  include  all  the  rays  that 
pass  through  the  objective,  the  extreme  marginal  ones 
as  well  those  that  are  more  central  and  image-forming. 
Thus  a  good  objective  in  my  possession,  claiming  an 
angular  aperture  of  135°,  which  it  has  when  it  is  meas- 
ured for  what  is  practically  the  angle  of  the  field,  has, 
when  measured  for  the  image-forming  rays,  an  angle  of 
only  about  96°  !  Still,  it  is  an  excellent  objective  and 
may  be  recommended;  yet  it  might  be  as  confidently 
commended  if  the  maker  had  told  the  truth  about  it,  or 
at  least  had  given  the  real  facts  in  the  case. 

The  method  of  measuring  this  true  angle  of  aperture 
does  not  materially  differ  from  that  already  described. 
The  only  essential  difference  is  that  the  objective  is 
focussed  on  an  object  while  it  is  being  measured,  and 


MICROSCOPICAL    PRAXIS.  I  2  "$ 

the  angle  is  read  as  soon  as  the  image  begins  to  be 
changed  for  the  worse.  The  procedure  demands 
rather  more  care  and  skill  than  that  already  described, 
but  the  owner  of  what  claim  to  be  wide-angled  objec- 
jectives  will  be  rewarded  if  he  teach  himself  the  neces- 
sary skill  for  his  own  personal  satisfaction.  The  mat- 
ter has  been  especially  studied  by  Dr.  George  E.  Black- 
ham,  of  Dunkirk,  N.  Y.,  who  has  called  the  attention  of  mi- 
croscopists  to  the  subject,  and  has  published  the  de- 
tails of  his  method  of  making  the  measurement.  The 
following  paragraphs  are  chiefly  from  Dr.  Blackham's 
paper. 

The  objective  to  be  measured  is  attached  to  the 
microscope  with  the  eye-piece  in  place,  exactly  as  for 
ordinary  work,  and  is  focussed  on  some  suitable  object 
in  the  centre  of  the  field,  the  correction  collar  being 
used  if  necessary  to  get  the  best  image  the  objective  is 
capable  of  giving.  The  object  should  be  a  transparent 
one,  the  resolution  of  which  is  a  fair  test  of  the  powers 
of  the  lens.  After  these  arrangements  have  been  com- 
pleted, the  body  of  the  microscope  is  turned  to  a  hori- 
zontal position,  the  mirror  swung  out  of  the  way,  and 
the  object  illuminated  from  below  the  stage  by  a  nar- 
row radiant,  such  as  the  flame  of  a  toy  candle.  The 
source  of  illumination  is  then  moved  to  the  right  and 
the  left  in  succession,  till  either  the  centre  of  the  field 
becomes  darkened  or  the  image  is  spoiled.  The 
angular  value  of  this  distance  through  which  the  source 
of  illumination  can  be  moved  before  this  takes  place,  is 
the  available  angular  aperture  of  the  lens;  that  is,  the 
useful  aperture  for  definition.  In  some  lenses  the 
image  is  spoiled  long  before  the  centre  of  the  field  7s 
darkened,  so  that  the  aperture,  for  the  mere  transmis- 
sion of  light,  is  much  greater  than  that  for  definition.  Such 


124  MICROSCOPICAL    PRAXIS. 

lenses  are  imperfectly  corrected  for  the  marginal  rays  and 
their  performance  can  be  improved  by  cutting  off  these 
aberrant  marginal  rays  by  means  of  diaphragms,  and  so 
reducing  their  angular  aperture  (for  transmission). 

Instead  of  moving  the  candle  as  Dr.  Blackham  sug- 
gests, the  microscope  may  be  turned  on  the  graduated 
base  which  some  stands  have,  or  the  angular  value  may 
be  ascertained  by  some  of  the  other  methods  previously 
described,  and  which  are  rather  easier  for  most  experi- 
menters than  to  make  the  estimates  and  calculations 
demanded  by  Dr.  Blackham's  method.  Or  if  the 
mirror-bar  be  graduated  and  capable  of  being  swung  to 
one  side,  as  Dr.  Blackham  says  it  should  be  (in  which 
we  all  agree  with  him),  the  microscopist  has  a  means  of 
reading  the  obliquity  to  which  it  is  swung  by  substitut- 
ing a  toy  candle  for  the  mirror  and  rotating  it  to  one 
side  instead  of  moving  the  candle  alone  as  in  the  other 
plan,  or  of  turning  the  microscope  on  its  base. 

If  greater  accuracy  be  desired,  it  can  be  secured  by 
the  use  of  an  opaque  slide  with  a  transparent  line 
across  it,  in  which  the  objects  are  mounted,  and  this 
line'can  be  so  placed  on  the  stage  as  to  bisect  the  field 
of  view  very  accurately  in  a  vertical  direction,  and  the 
exact  moment  at  which  the  centre  of  the  field  is 
darkened  can  thus  be  determined  with  greater  pre- 
cision. Such  a  slide  can  readily  be  made  by  cutting  a 
3x1  inch  slip  from  a  photographic  plate,  exposing  it  to 
the  full  sunlight,  developing  and  fixing  it,  and  then 
drawing  a  sharp  knife  across  it  on  the  film  side.  In 
the  transparent  line  thus  made,  diatoms  or  any  other 
objects  may  be  mounted  in  balsam  and  covered  in  the 
usual  way. 

The  plan  of  measurement  here  described,  if  used 
without  any  devices  below  the  stage,  can  measure  aper- 


MICROSCOPICAL    PRAXIS.  125 

tures  only  below  i.oo  numerical  aperture;  that  is  to 
say,  angular  apertures  of  180°  air,  of  97°  31'  water,  or 
of  82°  if  in  glass  or  homogeneous  immersion-fluid, 
and  must  always  give  the  results  in  terms  of  the  equiva- 
lent air-angle.  Hence  when  we  have  occasion  to 
measure  the  aperture  of  a  lens  exceeding,  or  indeed 
closely  approaching  i.oo  N.  A.,  it  becomes  necessary  to 
use  below  the  stage  some  device  like  the  little  hemis- 
pherical lens,  which  being  attached  to  the  under  side  of 
the  slip  by  an  immersion  contact,  allows  the  rays  to 
pass  into  the  slide  without  refraction,  and  consequently 
gives  the  angle  in  terms  of  the  glass,  or  the  homogene- 
ous angle. 

It  not  rarely  happens  that  the  manufacturing  optic- 
ian does  not  mark  on  the  mounting  of  his  objectives 
the  aperture  that  the  purchaser  thinks  should  be  there, 
after  the  purchaser  has  measured  it.  This  refers,  of 
course,  to  the  aperture  that  includes  all  the  entering 
light,  the  marginal  as  well  as  the  image-forming  rays. 
The  optician  will  never  use  half  a  degree,  as  there  is  no 
reason  why  he  should;  and  he  does  not  seem  to  care 
much  about  odd  numbers,  as  shown  in  the  following 
table  of  angular  and  numerical  aperture  of  the  objec- 
tives mentioned,  which  were  all  measured  with  an  Abbe 
apertometer  in  the  expert  hands  of  Prof.  M.  D.  Ewell, 
of  Chicago,  from  whose  report  the  list  is  taken. 


126 


MICROSCOPICAL    PRAXIS. 


Table   of  the   Aperture   of  Certain  Objectives. 


Maker. 

Bausch  &  Lomb, 

u  a 


Beck, 
Crouch, 

Grunow, 

Gundlach, 
Hartnack. 
Leitz,  4 


Spencer  &  Co., 

II  U 

«  « 

Spencer  &  Smith, 


Description. 

Professional  i  in. 
/4  in. 

2  in.,  first  class. 
^  in.,  students. 
-J-  in.,  students. 
^  in.,  first  class. 
-fa  in.,  first  class. 
^  in.,  horn.  imm. 
\  in.,  first  class, 
i  in. 


Aperture 
claimed 

Aperture 
as 

by  maker. 

measured. 

36° 

3°° 

60° 

61° 

22° 

21° 

75° 

7°° 

110° 

95/2° 

140° 

114° 

ucT        iu~ 

N.  A.  1.43    N.  A.  1.28 
100°  98° 

25°          24° 
140°  \  ^closed 
(  147    open. 

N.  A.  1.41    N.  A.  1.39 


f« 

•J-,  horn.  imm. 

i  in.  40°         47° 

18  mm.,  No.  3,  dry.  N.  A.  0.28  N.  A.  0.28 
32  mm.,  No.  7,  dry.  N.  A  0.85  N.  A.  0.88 
-fa  in.,  oil  imm.  N.  A.  1.30  N.  A.  1.28 

J  in.,  students. 
-fa  in.,  horn.  imm. 

-h  in->     " 


"  " 


94 

B.  A.   130°  N.  A.  1.36 
"  B.  A.  125°  -N.  A.  1.32 

-fa,  first  class.  130°       126° 

•J,   first  class,  dry.      150°        167° 


in,  horn.  imm.  B.  A.  138°  N.  A.  1.41 


Tolles, 


Zeiss, 


i  in. 

33           32i 

«           i  jj-j 

70°         71° 

"          ^  horn.  imm. 

N.  A.  1.  00  N.  A.  0.97 

-J-,    horn.  imm. 

N.  A.  1.32   N.  A.  1.31 

2  in. 

Unknown       i2\ 

y^in,  water  imm. 

u      j  0.99  closed 
|  0.90  open 

1,  A  A. 

•36°     3i° 

i,  c   c. 

o                       o 
90              102 

Y^-g,  horn.  imm. 

N.  A.  1.27   N.  A.  1.27 

MICROSCOPICAL    PRAXIS.  127 

To  Measure  Numerical  Aperture  (N.  A.). 

For  the  measurement  of  numerical  aperture,  the  late 
Dr.  Allen  Y.  Moore  used  a  method  whose  application 
was  probably  original  with  himself.  The  following  is 
his  description.  A  low-power  objective  (a  three-inch 
or  a  four-inch  is  convenient  for  the  purpose),  is  at- 
tached to  the  microscope,  and  the  latter  placed  in  a 
horizontal  position.  The  objective  whose  numerical 
aperture  is  to  be  measured  is  screwed  into  the  sub- 
stage  with  its  front  facing  toward  the  mirror  which 
should  be  turned  as  far  as  possible  to  one  side,  or 
preferably  removed  from  the  stand.  If  now  the  micro- 
scope be  properly  focussed,  a  bright  disk  of  light  will 
be  seen,  and  will  represent  the  acting  diameter  of  the 
back  lens  of  the  objective  in  the  sub-stage.  The 
camera  lucida  should  be  attached  to  the  eye-piece,  and 
the  diameter  of  this  disk  marked  on  paper.  A  hemi- 
spherical lens  is  now  to  be  applied  to  the  front  of  the  ob- 
jective to  be  measured,  and  should  have  immersion 
contact  with  it,  that  is,  should  be  attached  by  means  of 
a  drop  of  glycerine  or  of  homogeneous-immersion  fluid. 
This  will  enlarge  the  bright  circle  to  the  full  aperture 
of  the  objective,  and  this  bright  disk  should  also  be 
drawn  on  the  paper.  Remove  the  objective  from  the 
sub-stage,  and  on  the  microscope-stage  place  a  micro- 
meter ruled  to  thousandths  of  an  inch,  and  with  the 
camera  lucida  project  these  lines  upon  the  two  circles 
on  the  paper,  thus  measuring  their  diameter.  If  the 
diameter  of  the  larger  circle  be  divided  by  that  of  the 
smaller,  the  quotient  will  be  the  numerical  aperture 
This  applies  however,  only  to  objectives  whose  N.  A.  is 
greater  than  i.oo. 


128  MICROSCOPICAL    PRAXIS. 

Definition. 

The  definition,  or  the  defining  power,  of  an  objective, 
is  its  ability  to  produce  an  image  of  the  object  ex- 
amined that  shall  be  clear,  sharp,  crisp  and  true.  The 
outlines  of  the  image,  as  has  already  been  remarked, 
should  be  so  distinctly  defined  that  they  shall  be  as 
sharply  marked  as  the  lines  of  a  copper-plate  engrav- 
ing, and  the  structure  of  the  object  must  at  the  same 
time  be  as  clearly  defined  as  is  the  contour. 

The  greater  the  angular  aperture  the  more  perfect 
and  satisfactory  will  be  the  definition.  Small-angled 
objectives  have  some  qualities  that  make  them  more 
easily  manipulated  by  the  novice,  but  the  exquisite 
definition  of  the  wide-angled  glasses  is  not  one  of  them; 
and  the  wide-angled  lenses  lack  the  element  of  penetra- 
tion, or  depth  of  focus,  possessed  by  the  smaller-angled 
objectives  and  valued  to  a  certain  extent  in  some  in- 
vestigations. 

There  are  many  tests  for  the  definition  of  low  and  of 
medium  powers  of  small  angle.  For  the  former,  Mr. 
E.  M.  Nelson,  an  accomplished  British  microscopist, 
suggests  the  flattened  proboscis  of  the  blow-fly  mounted 
in  Canada  balsam,  and  for  the  latter  the  minute  hairs  on 
the  same  object.  Under  medium  powers  of  high 
angle,  he  recommends  stained  bacteria,  and  the  diatom 
Pleurosigma  formosa  mounted  in  balsam. 

According  to  Prof.  Abbe,  small  fragments  of  the  dia- 
tom Pleurosigma  angulatum  may  be  used  to  advantage 
as  tests  of  even  the  best  and  widest  angled  immersion 
objectives.  In  connection  with  the  illumination  during 
the  testing,  he  recommends  that  the  mirror  should  be 
so  arranged  that  the  light  shall  just  graze  the  centre  of 
the  front  surface  of  the  objective,  the  striae  of  the  dia- 
tom being  at  right  angles  to  the  general  direction  of 


MICROSCOPICAL    PRAXIS.  I2Q 

the  illumination;  this  tests  the  marginal  zone  of  the 
lens.  The  mirror  is  then  to  be  shifted  laterally  so  as  to 
produce  the  most  oblique  illumination.  In  both  cases 
the  outlines  and  the  structural  striae  should  not  only 
appear  equally  sharp,  but  should  coincide  without  dif- 
ference o(  level  and  without  lateral  displacement.  If 
an  objective  fulfills  these  requirements,  at  least  in  the 
centre,  says  Professor  Abbe,  it  may  be  depended  upon 
to  produce  accurate  images.  An  intermediate  position 
of  the  mirror  would  furnish  additional  proof  that  the 
different  zones  co-operate  simultaneously  in  the  pro- 
duction of  the  images. 


Resolving  Power. 

The  resolution,  or  the  resolving  power,  of  an  object- 
ive is  its  ability  to  separate  fine  and  close  markings  or 
lines,  such  "as  the  striae  on  the  diatoms,  or  those  closely 
ruled  lines  on  the  artificial  test-plates  of  various 
makers.  This  ability  depends  upon  certain  qualities  of 
the  lens,  especially  upon  the  numerical  aperture,  and  not 
upon  magnifying  power.  In  the  separating  of  very 
fine  and  close  lines  it  is  customary  to  use  high-power 
eye-pieces,  not  because  the  increase  in  magnifying 
power  increases  the  resolving  power  of  the  objective, 
but  because  it  separates  the  lines  a  little  wider  so  that 
the  eye  may  more  easily  see  them. 

10 


130  MICROSCOPICAL    PRAXIS. 

The  reader  should  remember  that  to  do  much  in  the 
resolving  of  diatoms  or  of  fine  rulings,  the  eye  must  be 
specially  educated  for  the  work.  Whilst  the  micros- 
copistmay  be  expert  in  the  use  of  his  objectives  and  il- 
luminating apparatus,  and  whilst  he  may  be  a  learned 
histologist,  or  an  accomplished  investigator  in  some 
other  department  of  science,  he  may  not  be  able  to  ex- 
hibit fine  lines  or  diatom-striae,  either  to  himself  or  to  his 
friends.  This  is  aspecial  department  in  which  special  eye- 
training  is  demanded.  And  whilst  it  is  well  to  be  able 
to  resolve  the  most  difficult  diatoms,  so  as  to  be  able  to 
know  positively  by  one's  own  experience  what  a  certain 
objective  will  do,  it  should  not  be  the  chief  aim  in  the 
microscopist's  life.  The  resolving  of  diatoms  is  useful 
in  studying  the  manipulation'  of  the  objective,  and  im- 
portant in  comparing  the  action  of  different  objectives 
from  different  makers,  and  with  different  angles  of 
aperture,  but  there  are  many  things  more  important 
than  these  "microscopical  gymnastics."  It  is  exceed- 
ingly important  that  the  owner  of  good  objectives 
should  study  them  to  learn  what  they  will  do,  and  how 
he  may  obtain  the  best  results  from  them,  but  to  know 
no  more  of  microscopical  research  than  is  contained  in 
an  attempt  to  force  one  objective  to  resolve  a  few  lines 
more  than  another,  seems  a  poor  compensation  for  the 
time,  labor  and  expense,  when  the  world  is  teeming 
with  innumerable  things  about  which  we  know  nothing, 
and  when  even  one  little  corner  of  one  little  field  of  in- 
vestigation, well  cultivated  with  the  microscope,  will 
bring  the  cultivator  fame,  and  the  scientific  world  in- 
crease of  happiness  through  increase  of  knowledge. 

The  resolving  power  of  a  well-corrected  objective  de- 
pends upon  its  numerical  aperture.  A  lens  of  wide 
aperture  will  show  finer  and  closer  lines  than  will  a  small- 


MICROSCOPICAL    PRAXIS.  131 

angled  glass,  the  resolution  increasing  and  diminishing 
in  a  certain  proportion  as  the  numerical  aperture  is  en- 
larged or  contracted,  all  objectives  that  have  the  same 
numerical  aperture  having  the  same  resolving  power. 

The  character  of  the  illumination,  whether  central  or 
oblique,  is  an  important  element.  Oblique  light  will 
bring  out  markings  not  visible  with  central  illumina- 
tion, and  the  objective  that  will  resolve  Pleurosigma 
angulatum  with  central  light,  other  things  being  equal, 
is  a  better  lens  than  the  one  that  resolves  it  only  with 
oblique  illumination.  « 

The  adjustment  of  the  objective  is  also  of  great  im- 
portance in  this  connection.  And  the  medium  in 
which  the  test  is  immersed  has  great  influence.  A  dia- 
tom mounted  dry  is  more  easily  resolved  than  the  same 
diatom  in  Canada  balsam.  The  character  of  the  lines 
or  of  the  dots  is  important  to  the  success  of  the  work. 
Two  diatoms  may  have  the  same  number  of  lines  to  the 
fraction  of  an  inch,  yet  one  may  be  easily  resolved, 
whilst  the  other  may  tax  the  resources  of  the  best  ob- 
jectives and  all  the  microscopist's  skill,  because  the 
striae  on  one  are  strongly  developed,  while  those  of  the 
other  are  faint,  low'and  delicate.  The  same  holds  true 
to  a  certain  extent  with  ruled  lines.  The  stronger  and 
deeper,  and  the  better  filled  with  graphite  these  cuts 
in  the  glass  may  be,  the  more  easily  are  they  resolved 
by  the  proper  objective,  or  at  least,  the  more  easily  are 
they  seen  by  the  eye. 

Although  diatoms  of  the  same  species  differ  to  some 
extent  in  the  character  and  often  in  the  number  of  their 
striae,  they  are  the  most  accessible  tests  for  resolu- 
tion that  we  have.  They  are  used  by  all  microscopists 
and  by  the  opticians,  as  they  form  a  ready  means  of 
comparison.  A  convenient  collection  is  Moller's  or 


132  MICROSCOPICAL    PRAXIS. 

Thum's  test-plate  (Probe  Platte)  consisting  of  a  slide  of 
twenty  diatoms  arranged  in  a  line  according  to  the  dif- 
ficulty of  their  resolution,  beginning  with  Triceratium 
favus  and  ending  with  Amphipleura  pellucida.  The  slides 
may  be  had  balsam-mounted  or  dry.  These  prepara- 
tions are  somewhat  costly,  and  the  reader  may  as  read- 
ily and  at  less  expense  supply  himself  with  several 
slides  of  different  tests,  with  the  advantage  of  having 
many  of  the  same  form  on  the  slide  in  many  different 
positions,  and  probably  of  different  degrees  of  fine  stri- 
ation.  0 

The  microscopist  that  desires  to  test  his  objectives, 
and  to  train  his  eye  to  see  fine  lines  and  dots,  should 
purchase  balsam-mounted  slides  of  Pleurosigma  angula- 
tum,  Frustulia  Saxonica,  Surirella  gemma,  Amphipleura 
pellucida,  or,  if  he  studies  convenience  rather  than  ex- 
pense, a  test  plate  by  Moller  or  by  Thum.  But  with 
the  diatoms  mentioned  he  will  have  enough  to  test  the 
best  and  highest-angled  objectives.  Pleurosigma  angu- 
latum  may  serve  as  a  test  for  the  one-fourth  or  the  one- 
fifth  inch  objective,  which  should  resolve  the  transverse 
lines  with  central  light,  and  three  sets,  one  transverse 
.and  two  oblique  in  opposite  directions,  under  the  proper 
illumination.  A  four-tenths  inch  by  Mr.  Tolles  in  my 
possession  will  resolve  this  diatom  in  beads  when 
mounted  in  styrax,  as  well  as  exhibit  the  transverse 
striae  on  a  dry  Surirella  gemma. 

Frustulia  Saxonica  is  more  difficult,  that  is,  the  striae 
are  finer  and  much  closer 'together  than  those  of  P.  an- 
gulatum,  demanding  a  high-power  of  wide  angle  to  re- 
solve them.  A  one-sixth  or  one-eighth  of  fine  quality 
will  be  needed  to  conquer  them. 

Amphipleura  pellucida  is  the  most  difficult  of  the 
known  diatom  tests.  Photography  has  recently  resolved 


MICROSCOPICAL    PRAXIS.  133 

it  into  dots  or  beads,  but  to  show  the  exceedingly  fine 
and  delicate  transverse  striae  only,  is  a  labor  for  the 
best  of  ordinary  modern  objectives,  although  the  ma- 
jority of  the  recent  lenses  of  wide  aperture  will  do  the 
work  easily,  if  properly  treated.  The  diatom  has  been 
resolved  into  beads  by  Powell  and  Lealand's  -J-  inch  ob- 
jective, and  by  the  remarkable  new  apochromatic  y1^, 
1.63  N.  A.  by  Zeiss,  in  the  hands  of  Dr.  H.  Van  Heurck. 

Oblique  light  is  always  necessary  for  this  kind  of 
work;  it  is  often  needed  so  oblique  that  the  upper  edge 
of  the  mirror,  when  swung,  to  one  side,  is  only  a  little 
below  the  surface  of  the  stage.  The  concave  mirror 
should  be  carefully  focussed  on  the  object,  and  the  dia- 
tom so  placed  that  the  striae  desired  to  be  shown  shall 
be  at  right  angles  to  the  direction  of  the  illumination, 
and  the  position  of  the  adjustment-collar  must  be  care- 
fully and  frequently  changed  until  the  best  results  are 
attained.  Daylight  is  not  adapted  to  suoh  work.  Di- 
rect sunlight  should  never  be  used. 

In  the  study  of  the  striae  of  diatoms  the  mirror  alone 
is  often  not  sufficient.  In  all  such  work  the  hemispher- 
ical lens  is  a  useful  accessory,  concentrating  the  light 
on  the  diatom,  and  adding  somewhat  to  its  intensity. 
This  lens  is  simply  a  little  button  of  glass  about  half  an 
inch  in  diameter,  and  of  the  proper  curvature  to  focus 
the  light  upon  the  object.  It  is  attached  to  the  lower 
surface  of  the  slide  by  a  drop  of  water,  of  glycerine  or 
of  homogeneous-immersion  fluid.  If  too  much  liquid 
be  used  the  lens  will  slip  out  of  position  when  placed 
on  the  inclined  stage.  Use  only  enough  therefore  to 
hold  it  firmly  and  to  expel  all  the  air  from  between  it 
and  the  slide  to  which  it  is  attachedr 

The  following  table  gives  a  list  of  the  diatoms  on 
Holler's  test-plate,  with  the  number  of  striae  to  the 


134  MICROSCOPICAL    PRAXIS. 

inch.     Eupodiscus  argus  begins  and  ends  the  line  so  that 
it  may  be  readily  found. 

Diatom.  Striae  in  i-iooo  inch. 

1.  Triceratium  favus  Ehr.  3.1  to    4.0 

2.  Pinnularia  nobilis  Ehr.  11.7  to  14.0 

3.  Navicula  lyra  Ehr.  var.  14.5  to  18.0 

4.  Navicula  lyra  Ehr.  23.0  to  30.5 

5.  Pinnularia  interrupta  Sm.  var.  25.5  to  29.5 

6.  Stauroneis  Phcenicenteron  Ehr.  31.0  to  36.5 

7.  Grammatophora  marina  Sm.  -  36.0  to  39.0 

8.  Pleurosigma  Balticum  Sm.      -  32.0  to  37.0 

9.  Pleurosigma  acuminatum  (Kg.)  Gr.  41.0  to  46.5 

10.  Nitzchia  amphioxys  Sm.  43.0  to  49.0 

11.  Pleurosigma  angulatum  Sm.  44.0  to  49.0 

12.  Grammatophora  oceanica  Ehr.  60.0  to  67.0 

13.  Surirella  gemma  Ehr.      -  43-°  to  54.0 

14.  Nitzchia  sigmoidea  Sm.  -  61.0  to  64.0 

15.  Pleurosigma  fasciola  Sm.  var.  55.0  to  58.0 

16.  Surirella  gemma  Ehr.      -  64.0  to  69.0 

17.  Cymatopleura  elliptica  Breb.     -  55.0  to  81.0 

1 8.  Navicula  crassinervis  Breb.     -  78.0  to  87.0 

19.  Nitzchia  curvula  Sm.       -  83.0  to  90.0 

20.  Amphipleura pellucida  Kg.       -  92.0  to  95.0 
The  following  is   the   list    of   the    diatoms   used    on 

Thum's  test-plate. 

1.  Triceratium  favus  Ehr. 

2.  Navicula  nobilis  Kutz. 

3.  Navicula  lyra  Ehr.  var. 

4.  Navicula  lyra  Ehr. 

5.-  Pleurosigma  attenuatum  Sm. 

6.  Stauroneis  Phamic  enter  on    Ehr. 

7.  Grammatophora  marina  Sm. 

8.  Pleurosigma  Balticum  Sm. 

9.  Pleurosigma  acuminatum  (Kg.)  Gr. 


MICROSCOPICAL    PRAXIS.  135 

10.  Plettrosigma  angulatum  Sm. 

11.  Nits chia  sigma  Sm. 

12.  Surirella  gemma  Ehr. 

13.  Nits  chia  sigmodea  Sm. 

14.  Nitschia  obtusa  var.  Schweinfurthii  Gr. 

15.  Cymatopleura  nobilis  Hantz. 

1 6.  Nitschia  linear  is  Sm. 

17.  Grammatophora  subtilis  Bail. 

1 8.  Surirella  gemma  Ehr. 

19.  Frustttlia  Saxonica  Rahb. 

20.  Amphipleura  pellucida  Kg. 

In  addition  to  diatoms,-  microscopists  are  in  the 
haoit  of  using  artificial  tests  in  the  form  of  exceedingly 
fine  and  close  lines  ruled  on  glass  by  a  special  machine. 
The  most  celebrated  of  these  test-plates,  those  of  the 
late  F.  A.  Nobert,  of  Prussia,  have  already  been  referred 
to,  the  best  known  and  rnqst  important  being  his  slide 
bearing  nineteen  bands  of  lines,  the  number  of  lines  in 
each  band  increasing  and  the  spaces  decreasing  up  to 
the  last  or  nineteenth.  The  other  plates  by  the  same 
maker  are  seldom  heard  of  at  the  present  day. 

The  Lepisma  saccharina,  the  markings  on  whose 
larger  scales  are  as  close  together  as  the  lines  on  the 
first  band  of  this  plate,  is  a  common  little  insect  often 
seen  in  old  books  or  running  actively  in  dark  places. 
It  is  so  elongated  and  flattened,  and  so  silvery-gray  in 
color,  that  it  is  frequently  called  the  "silver-fish  insect." 
The  body  has  three  bristle-like  radiating  caudal  fila- 
ments and  two  thread-like  antennae,  and  when  full- 
grown,  may  be  fully  an  inch  long.  It  is  covered  with 
scales  which  at  one  time  were  used  as  tests,  and  are 
still  commendable  for  certain  purposes.  They  should 
be  mounted  dry. 


136  MICROSCOPICAL    PRAXIS. 

The  following  is  Naegeli  and  Schwendener-'s  list  of 
diatoms  whose  lines  correspond  in  number  to  the  num- 
ber of  lines  in  each  band  of  Nobert's  nineteen-band 
plate. 

Band.  Lines  to  o.ooi  in.  Lines  to  mm.  Approximate  equivalent. 

1.  11.26         443        Lepisma  saccharina;  large  scales. 

2.  16.89         665       Pinnularia  viridis. 

3.  22.52         886 

4.  28.13  4108 

5-         33-78       1329       Pleurosigma  Balticum. 

6.  39.41        1550  attenuatum. 

7.  45.04       1773 

9.'  56.*30  22?6  \Ptiurosigma  angulatum. 

10.  61.93  2439       Grammatophora  marina. 

11.  67.56  2653  Nitzchia  lima ris. 

12.  73.19  2870  \Navicularhomboides. 

13.  78.82  3105  \  Grammatophora  subtilissima. 

14-  84.45        3322 

15-  90-08       3546     \  Frustulia  Saxonica. 

16.  95-71       3768     r 

17.  101.34       3989     )  Amphipleura pellucida. 

18.  106.97       4211 

19.  112.60       4433 

In  "The  English  Mechanic,"  Mr.  G.  D.  Hirst  de- 
scribes a  method,  not  original  with  him,  of  intensify- 
ing the  resolving  power  of  an  objective  when  used 
over  close-lined  tests.  He  says:  Take,  for  instance, 
the  diatom  Amphipleura  pellucida,  and  having  got  the 
best  results  obtainable  with  the  illuminating  apparatus 
at  one's  disposal,  let  the  analysing  prism  of  the  polari- 
scope  be  placed  over  the  eye-piece,  and  rotated  until 
it  darkens  the  field,  which  it  will  do  although  not  to  the 
same  extent  a's  when  used  with  the  polarizing 


MICROSCOPICAL    PRAXIS.  137 

prism.  On  carefully  focussing  the  diatom,  the  lines 
will  show  themselves  with  an  extraordinary  increase  of 
definition.  Specimens  that  without  the  aid  of  the 
prism  show  only  a  washy  sort  of  resolution,  will  now 
show  the  lines  as  black  as  the  bars  of  a  gridiron. 

The  application  of  the  prism  will  of  course  not  make 
an  objective  resolve  a  test  beyond  the  reach  of  its  aper- 
ture. 


Penetration. 

The  penetrating  power  of  an  objective  is  its  ability 
to  show  the  structure  of  an  object  below  the  plane 
for  which  it  is  focussed;  that  is,  the  objective  can  pene- 
trate to  a  certain  distance  into  the  object  without  being 
lowered  by  the  focussing  mechanism.  It  is  of  very 
little  importance,  and  is  incompatible  with  resolving 
power.  It  depends  chiefly  upon  the  numerical  aper- 
ture, like  so  many  other  qualities  of  the  objective,  de- 
creasing in  a  certain  proportion  with  the  increase  of  the 
aperture,  and  increasing  as  that  diminishes. 

One  of  the  advantages  of  objectives  with  wide  aperture 
(N.  A.)  is,  that  while  they  show  the  most  delicate  de- 
tails of  the  surface  for  which  they  are  focussed,  there  is 
no  confusion  by  the  introduction  of  a  somewhat  indistinct 
view  of  structure  below  that  focal  plane,  yet  the  objec- 


138  MICROSCOPICAL    PRAXIS. 

tive  may,  in  certain  conditions,  be  depressed  for  the 
satisfactory  examination  of  an  inferior  plane  of  the  ob- 
ject, with  little  interference  with  the  superior  surface 
just  examined  and  left.  With  certain  wide-angled 
glasses  it  is  possible  to  produce  optical  sections  of  the 
object,  by  which  the  relations  of  structure  may  be  satis- 
factorily demonstated. 


Working-distance. 

The  working-distance  of  an  objective  is  the  distance 
between  the  lower  surface  of  the  front  lens,  when  the 
objective  is  focussed,  and  the  upper  surface  of  the  ob- 
ject, the  practicable  distance  being  reduced  by  the  pres- 
ence of  the  cover-glass.  It  varies  according  to  the 
magnifying  power  and  to  the  angular  aperture,  depend- 
ing chiefly  upon  the  latter.  The  optician  can  control  it 
to  a  certain  extent,  making  it,  within  certain  limits,  long 
or  short  at  will,  since  it  depends  not  only  on  the  power 
and  on  the  aperture,  but  upon  the  thickness  of  the 
lenses  composing  the  objective,  upon  their  curvature 
and  their  number. 

Objectives  are  not  named  according  to  their  work- 
ing-distance. A  one-inch  objective  will  not  have  a  one- 
inch  working-distance,  and  the-  latter  will  not  be  one- 
tenth  of  an  inch  with  a  one-tenth  inch  microscope-lens 
Objectives  are  rated  according  to  their  power  and  their 


MICROSCOPICAL    PRAXIS.  139 

focal  length  in  comparison  with  single  lenses.  A  one- 
inch  objective  is  supposed  to  have  the  same  magnifying 
power  as  a  simple  lens  of  one-inch  focus,  and  a  one- 
fifth  inch  objective  to  be  the  same  in  power  as  a  simple 
lens  having  a  focal  length  of  one-fifth  inch. 

Working-distance  is  a  convenient  quality,  since  an 
objective  of  long  working-distance  is  more  easily  used 
than  one  that  focusses  so  close  to  the  cover  as  to  be 
almost  in  contact  with  it.  In  some  cases  however,  it  is 
an  obstacle,  especially  in  certain  immersion-objectives 
where  the  optician  has  so  increased  it  that  the  capillary 
attraction  is  scarcely  sufficient  to  hold  the  immersion 
liquid  in  place  between  the  lens  and  the  cover.  But 
this  is  not  a  common  occurence. 


To  Measure  Working-distance. 

If  the  stand  have  a  scale  and  vernier  at  the  side  of 
the  body,  and  the  front  lens  of  the  objective  is  flush 
with  the  mounting,  the  working-distance  may  be 
readily  measured  by  carefully  bringing  the  objective  in 
contact  with  the  slide,  after  which  it  is  focussed  on  any 
imperfections  or  small  particles  on  the  surface,  and  the 
distance  to  which  it  has  been  raised  is  then  to  be  read 
on  the  scale.  In  high-power  objectives  the  distance 
may  be  ascertained  by  the  fine-adjustment  screw,  if  the 
milled-head  is  graduated  and  the  lens  flush  with  the 


140  MICROSCOPICAL    PRAXIS. 

mounting,  as  it  often  is  not.  Yet  the  distance  between 
the  front  of  the  mounting  and  the  front  of  the  lens  is  so 
short  that  it  may  be  disregarded  in  the  lower  powers. 
In  the  higher,  if  a  more  accurate  measurement  is  de- 
sired, it  may  be  made  by  using  two  objectives. 

In  such  cases,  place  the  lens  whose  working  distance 
is  to  be  ascertained,  in  the  substage  so  that  the  front 
faces  upward,  and  arrange  an  object  behind  it  until  the 
small  image  is  distinct  at  its  focus.  With  a  low-power 
objective  on  the  microscope,  focus  on  the  face  of  the 
objective  to  be  examined,  and  measure  the  distance 
through  which  the  body-tube  must  be  raised  in  order  to 
get  a  distinct  view  of  the  image  formed  by  the  objec- 
tive whose  working-distance  is  to  be  ascertained. 

When  the  working-distance  of  an  adjustable  objec- 
tive is  to  be  ascertained,  the  collar  should  be  at  or  near 
closed  point,  unless  the  microscopist  desires  to  know  it 
both  for  covered  and  for  uncovered  objects,  as  it  varies 
slightly  under  these  conditions  in  which  case  he  must 
measure  it  with  the  collar  at  both  adjustments. 

It  varies  also  not  only  with  every  adjustment,  since 
every  movement  of  the  collar  changes  the  focus,  but 
also  according  teethe  eye-sight  of  the  observer,  and  the 
power  of  the  eye-piece.  A  near-sighted  microscopist 
must  focus  his  objectives  nearer  the  object,  and  thus 
shorten  the  working-distance,  while  the  presbyopic  ob- 
server usually  lengthens  it,  especially  when  using  lenses 
of  low  power.  With  high-powers  the  same  fact  holds 
good,  but  not  to  so  great  an  extent. 


MICROSCOPICAL    PRAXIS.  141 

Magnifying  Power. 

The  magnifying  power  of  the  microscope  depends 
upon  the  power  of  the  objective  and  that  of  the  eye- 
piece, and  upon  the  length  of  the  body-tube.  It  is 
ascertained  by  multiplying  the  power  of  the  objective 
by  the  power  of  the  ocular,  the  length  of  the  body-tube 
being  assumed  to  be  of  the  standard  length  of  ten 
inches.  If  the  objective  alone  magnifies  fifty  diame- 
ters and  the  eye-piece  five,  the  power  of  the  combina- 
tion will  be  two-hundred  and  fifty  diameters.  This  is 
absolutely  correct  however,  only  when  what  is  called 
optical  tube-length,  which  is  a  very  variable  quality, 
has  been  taken  into  consideration.  For  the  unchanging 
ten-inch  body-tube  it  is  not  strictly  accurate,  yet  it  is 
near  enough  for  all  practical  purposes. 

The  power  of  the  objective  depends  upon  its  focal 
length,  both  increasing  and  diminishing  together,  while 
that  of  the  microscope,  or  combination  of  objective  and 
eye-piece,  depends,  in  addition,  upon  the  power  of  the 
•ocular  and  the  length  of  the  body-tube,  the  longer  the 
tube  or  the  stronger  the  eye-piece,  the  greater  the  am- 
plification. The  power  of  the  objective  is  also  affected 
by  the  adjustment  collar,  increasing  as  the  adjustment 
approaches  the  closed  point  and  becoming  greatest  at 
that  point. 

If  the  objective  is  correctly  named  according  to  its 
focal  length,  its  power  will  be  the  quotient  of  ten 
inches  (the  arbitrary  distance  of  distinct  vision,  and  the 
length  of  the  body-tube),  divided  by  the  focal  length; 
M  (magnifying  power)  =~,  where  /  is  the  focal  length. 
Thus  a  one-inch  objective  should  magnify  ten  diame- 
ters without  the  eye-piece;  a  one-fifth,  10-^-0.2  or  fifty 
diameters;  a  one-tenth,  one-hundred  diameters. 

But  all  objectives  are   not    correctly  named.     Some 


14?  MICROSCOPICAL    PRAXIS. 

opticians  in  their  nomenclature  under-rate  their  pro- 
ductions, so  that  a  lens  marked  J  by  one  maker  may 
in  reality  be  a  -^,  and  may  do  as  much  as  or  more  than 
a  one-fifth  by  another,  to  the  honor  and  glory  of  the 
former  apparently  lower  power.  Such  nomenclature  is 
not  only  dishonest,  but  it  is  inconvenient  and  mislead- 
ing when  we  wish  to  compare  two  objectives  by 
different  makers,  but  nominally  of  the  same  focal 
length.  Opticians  have  been  improving  in  this  respect 
of  late  years.  Formerly  the  complaints  were  many 
and  bitter,  and  the  opticians  answered  back,  defending 
themselves  or  trying  to  do  so,  and  thus  making  things 
lively.  But  the  makers  seem  to  have  taken  warning. 
At  least  the  complaints  have  become  fewer. 

To  measure  the  power  of  the  objective,  it  is  neces- 
sary to  have  a  point  from  which  to  begin  the  measure- 
ment, and  this,  it  is  said,  should  be  the  optical  centre, 
or  "that  point  in  a  lens  through  which  if  a  ray  passes,  it 
enters  and  emerges  in  parallel  lines;"  but  since  nobody 
knows  where  the  optical  centre  may  be,  and  since  some 
objectives  have  none,  it  is  rather  difficult  to  find.  Yet 
when  it  exists,  it  may  be  discovered  by  going  through 
with  three  pages  of  mathematical  calculations  to  get 
two  or  three  formulae  which,  when  combined  and 
solved,  will  give  the  point  sought,  and  which,  when  ob- 
tained will  be  practically  worthless. 

A  method  which  is  sometimes  more  convenient  is  to 
measure  from  the  posterior  principal  focus  of  the  ob- 
jective, and  since  this,  in  high-powers  especially,  is 
close  to  the  surface  of  the  back  lens,  the  back  lens  may 
be  taken  as  the  starting  point. 

To  obtain  the  distance  of  this  posterior  focus  from 
the  upper  end  of  the  body,  prepare  a  paper  tube  closed 
at  one  end  with  a  translucent  diaphragm,  and  slip  this 


MICROSCOPICAL    PRAXIS.  143 

down  the  body  until  a  small  spot  of  light  is  sharply 
defined  in  the  centre  of  the  diaphragm.  Where  this 
spot  is  formed  is  said  to  be  the  posterior  focal  point  of 
the  objective.  Mark  the  place  on  the  outside  of  the 
body,  and  measure  the  tube-length  from  it.  With  low 
powers  the  length  of  the  body-tube  may  be  ten  inches 
from  the  front  lens,  since  the  posterior  focus  in  such 
lenses  is  higher  up  the  body,  for  which  the  length  of 
the  lens-mounting  will  make  approximate  compensa- 
tion. 

In  the  experiment  with  the  paper  diaphragm  the  tube 
should  be  small  enough  to  enter  the  mounting  of  the 
objective,  as  its  posterior  focus  is  often  close  to  the 
back  lens,  frequently  less  than  -^  inch  above  it,  when 
it  is  outside  of  the  lenses;  sometimes  it  is  somewhere 
inside  of  the  objective,  in  which  case  this  experiment 
will  fail,  as  the  paper  diaphragm  ca'nnot  then  reach  the 
focal  point.  To  be  strictly  accurate  in  these  matters  is 
difficult,  often  impossible  to  any  but  the  opticians,  and 
it  is  the  optical  tube-length  that  should  be  ascertained, 
yet  this  is  a  varying  quantity  with  every  combination 
of  objective  and  eye-piece,  since  it  is  the  distance 
between  the  posterior  principal  focus  of  the  objective 
and  the  anterior  principal  focus  of  the  ocular.  The 
former  it  is  often  impossible  to  find  in  any  practical 
way;  the  latter  is  usually  at  or  near  the  diaphragm 
within-  the  eye-piece  tube.  For  all  practical  purposes, 
however,  it  will  be  amply  sufficient  to  measure  the  dis- 
tance from  the  back  lens  of  the  objective  to  the  dia- 
phragm of  the  eye-piece  used,  both  positions  being  ob- 
tained without  difficulty,  as  the  diaphragm  of  the  ocular 
is  always  fixed  at  the  proper  point  by  the  optician. 

The  position  of  the  posterior  focus  of  the  objective 
depends  upon  the  construction  given  the  lens  by  its 


144  ( MICROSCOPICAL    PRAXIS. 

manufacturer.  Sometimes  it  is  outside  of  the  combina- 
tion and  can  then  be  ascertained;  often  it  is  some- 
where within  the  objective,  when  we  have  no  conven- 
ient means  of  getting  at  it.  The  late  W.  H.  Bulloch,  of 
Chicago,  paid  some  attention  to  the  subject,  examining 
several  objectives  by  different  makers,  his  purpose 
being  to  discover  a  practicable  method  which  might  be 
employed  by  the  microscopist  that  is  not  a  manufactur- 
ing optician,  nor  an  expert  in  the  application  of  micro- 
scopical optics.  His  conclusion  was  that  the  use  of 
the  posterior  principal  focus  of  the  objective  was  not 
possible,  for  the  reasons  already  detailed. 

His  method  of  ascertaining  this  focal  plane  was  to 
place  the  objective  to  be  examined  in  the  sub-stage  of 
a  microscope,  with  the  front  lens  directed  away  from 
the  microscope-stage.  He  then  used  a  low-power  ob- 
jective on  the  body-tube  to  find  the  position  of  an 
image  of  a  distant  object  formed  by  the  lens  in  the  sub- 
stage.  I  find  much  difference,  he  says,  among  the 
objectives  of  different  makers;  of  those  of  about  the 
same  magnifying  power,  some  have  the  posterior  focus 
within  the  combination,  others  have  it  some  distance 
behind  the  back  system;  for  example,  in  the  Spencer 
two-thirds  inch  of  36°  the  posterior  focus  was  found  to 
be  0.18  inch  within  the  combination,  measuring  from 
the  back  lens;  in  another  two-thirds  of  30°,  0.34  inch 
behind  the  posterior  combination. 

These  are  short  distances,  but  in  work  that  is  to  be 
followed  by  mathematically  accurate  results,  they  afe 
important;  yet  for  the  amateur  microscopist  they  may 
be  disregarded,  and,  as  has  been  said,  the  back  lens  of 
an  objective  higher  in  power  than  the  two-inch  may  be 
taken  as  the  starting  point  for  our  measurements. 

The  following  table  is  compiled  from  Mr.  Bulloch's 


MICROSCOPICAL    PRAXIS. 


145 


paper,  and  is  here  given  for  the  convenience  of  those 
possessing  the  objectives  mentioned,  and  also  to  show 
that  the  distances  are  so  short  that  they  be  disregarded 
by  all  except  perhaps  the  professional,  manufacturing 
optician. 


Maker  of 

Name  of 

Angle  of      Posterior  focus    Posterior 

Objective. 

Objective. 

Aperture,     from  front  sur-    focus  in- 

face.                    side  or  out- 

side  the 

combination 

Spencer, 

-^  horn.  imm. 

1.35  N.  A.     0.145         inside 

tt 

TV      "           " 

1.27  N.  A.     c.ii               " 

tt 

A  dry 

0   not  found  at 
imm.    170     open    point: 
atclosedo.25 

" 

i  dry 

115°     0.15        outside 

tt 

/-o  0.18  inside  of 

36     back. 

" 

i   inch 

o  0.02  inside  of 
40     back. 

" 

2        " 

0.67                      " 

Gundlach, 

-j*g-  imm. 

105°     0.15           inside 

" 

|  horn.  imm. 

1.40  N.  A.     0.53               " 

" 

-J-  dry 

135°     o-43 

Bausch  &  Lomb,  -J-  dry 

140°     0.41               " 

4< 

^  imm. 

1  80°  not  found 

a 

idry 

no0     0.49        outside 

tt 

T4o 

0.15           inside 

" 

i  inch 

_f~o  0.04  inside  of 
36     back. 

" 

J-  inch 

98°     0.02        outside 

Grunow, 

t     . 

0.85 

" 

ij 

0.123 

Zeiss, 

HDD)  dry 

116°     0.41         inside 

*' 

i  (c  c) 

90°     0.32            " 

•• 

t  (A  A) 

20°        0.84                    " 

" 

A*  closed 

8.53      outside 

" 

"    at  5 

7.8 

" 

"    open 

7.3 

Queen  &  Co., 

2  inch 

10°     1.57 

ii 

146  MICROSCOPICAL    PRAXIS. 

The  posterior  principal  focus  of  Zeiss's  variable  low- 
power  objective  A*,  and  that  of  Queen  &  Co.'s  2  inch, 
are  so  far  outside  of  the  combination  of  lenses  forming 
the  objective  that  the  reader  may  measure  the  distance 
for  his  own  satisfaction.  For  this  purpose  a  bright 
light  is  needed,  either  sunlight  or  the  parallel  rays  from 
the  Acme  lamp,  from  the  Stratton  Illuminator,  or  light 
from  a  lamp-flame  made  parallel  by  the  interposition  of 
a  bull's-eye  lens.  The  objective  is  then  held  with  the 
front  lens  toward  the  source  of  illumination,  which 
should  be  placed  at  the  opposite  side  or  end  of  the 
room,  and  the  light  focussed  on  the  wall.  The  focal 
point  will  be  represented  by  a  minute,  circular  dot  of 
light  of  great  intensity  and  purity,  the  little  spark  being 
smaller  than  the  head  of  the  smallest  pin.  By  moving 
the  objective  toward  or  from  the  wall,  the  exact  focus 
may  be  readily  obtained,  as  a  slight  movement  will  be 
sufficient  to  increase  the  size  of  the  spot,  and  greatly  to 
decrease  its  brilliancy.  The  distance  from  the  front 
surface  of  the  front  lens  to  the  wall,  will  be  the  dis- 
tance of  the  posterior  principal  focus.  This  distance 
cannot  be  obtained  in  this  way  with  any  microscope  ob- 
jective higher  in  power  than  the  two-inch. 

As  it  is  not  always  possible,  and  not  often  conven- 
ient, to  ascertain  the  optical  tube-length  by  actual 
measurement,  Mr.  A.  Ashe  has  disc9vered  a  method  of 
learning  this  optical  distance  by  a  simple  act  of  obser- 
vation and  an  equally  simple  calculation.  By  this  plan 
the  microscopist  may  know  what  optical  tube-length-  he 
has  in  practical  use  without,  as  Mr.  Ashe  remarks,  put- 
ting his  finger  at  certain  points  of  the  body-tube  and 
saying,  Here  is  the  posterior  principal  focus  of  the  ob- 
jective, and  here  the  anterior  principal  focus  of  the  eye- 
piece, and  then  measuring  the  distance  between  them 


MICROSCOPICAL    PRAXIS.  147 

by  a  foot-rule.  To  know  this  optical  distance,  how- 
ever, is  always  of  great  interest,  often  of  much  import- 
ance. For  instance,  by  Mr.  Ashe's  admirable  method, 
*  I  have  learned  that  with  a  certain  combination  of  ob- 
jective and  ocular  the  actual  optical  tube-length  is 
only  a  single  inch,  a  fact  that  explains  certain  features 
in  connection  with  this  special  objective  that  were 
otherwise  more  than  obscure. 

Mr.  Ashe's  method  is,  in  his  own  words,  as  follows. 
A  careful  estimate  is  made  of  the  power  of  the  micros- 
cope with  the  draw-tube  pushed  home  as  far  as  it  will 
go;  then  having  determined  this,  the  eye-piece  is  with- 
drawn three  or  four  inches,  the  exact  amount  being 
noted  and  the  increased  power  of  the  instrument  re- 
measured. 

We  are  now  in  possession  of  all  the  data  necessary  to" 
calculate  —  not  the  actual  optical  tube-length,  but  its 
arithmetical  equivalent  —  a  distinction  to  be  observed, 
though  the  difference  is  immaterial  to  the  purpose  in 
view. 

As  it  is  a  rule  in  optics  that  the  relative  sizes  of  im- 
ages formed  by  a  lens  at  different  points  in  its  axis  are 
in  strict  proportion  to  the  distance  of  those  points  from 
the  focus  of  the  lens,  we  may  arrange  the  following 
formula: 


where  A  is  the  amplification  of  the  instrument  with  the 
draw-tube  closed;  B  the  distance  to  which  the  eye-piece 
has  been  withdrawn;  C  the  increase  in  power  produced 
by  the  effect  of  B.  D  is  therefore  the  equivalent  of  the 
distance  separating  the  focus  of  the  objective  from  the 
anterior  focal  plane  of  the  ocular. 

To  illustrate  this  simply,  suppose  an  instrument  mag- 


148  MICROSCOPICAL    PRAXIS. 

nifies  100.  and  that  on  withdrawing  the  eye-piece  three 
inches,  the  power  is  found  to  be  increased  to  130,  the 
equivalent  of  the  tube-length  will  be,  by  the  foregoing 
rule,  5^p=IO,  or  ten  inches. 

A  simplification  of  this  method  has  recently  been 
recommended  by  its  author.  It  is  as  follows: 

Instead  of  measuring  the  power  of  the  microscope 
twice  over,  it  is  sufficient  to  place  a  micrometer,  or 
other  divided  scale,  on  the  stage,  and  to  count  the  num- 
ber of  lines  that  fill  the  field  of  view  from  side  to  side, 
then  to  pull  out  the  draw-tube  some  inches  and  repeat 
the  counting. 

Of  course  the  greater  the  increase  in  power  the  fewer 
will  be  the  number  of  lines  seen.  In  other  words,  the 
number  of  lines  and  the  magnifying  power  are  in  inverse 
proportion  to  each  other. 

Now.  for  the  purpose  in  view,  it  does  not  matter  one 
iota  what  the  actual  powers  of  the  instrument  may  be, 
with  its  draw-tube  in  various  positions,  so  long  as  we 
know  the  proportion  those  powers  bear  to  each  other, 
.and  this  proportion  we  shall  find  in  the  relative  number 
of  lines  which  fill  the  field  of  view  at  the  same  points. 

Hence  (bearing  in  mind  the  inversion  of  the  ratio) 
we  may  look  upon  the  number  of  lines  counted  as 
though  they  were  the  actual  powers  of  the  microscope, 
and  proceed  at  once  to  apply  the  formula,  thus  obtain- 
ing results  which  correspond  precisely  with  those  given 
by  the  more  lengthy  process. 

An  example  may  be  useful: — 

Magnifying  power  with  the  tube  closed,  100 

Magnifying  power  with  the  tube  extended  three 

inches,    -  X50 

Increase,  5° 


MICROSCOPICAL    PRAXIS.  149 

Therefore,  I-^—6  (inches),  optical  tube-length. 
Again  by  the  shorter  way: — 
Number  of  lines  that  fill  the  field  with  the  tube 

closed,  30 

Number  of  lines  that  fill  the  field  with  the  tube 

extended  3  inches,  20 


Increase,  10 

Therefore,  inverting  these,   ~  =  6    (inches),  optical 
tube-length. 

An  objective  well  corrected  for  central  light  is  not 
necessarily  well  corrected  for  oblique  light,  and  vice 
versa.  This  is  especially  noticeable  with  the  best,  wide- 
angled,  dry  objectives,  although  it  holds  true  with  the 
best  of  any  kind,  although  probably  not  to  the  same 
extent  as  with  the  widest  angled  dry  lenses.  Certain 
immersion-fluids  also  demand  a  different  adjustment  of 
the  objective  with  central  and  with  oblique  light,  and 
again  vice  versa,  all  these  delicate  little  points  in  the 
manipulation  of  his  best  tools  being  the  task  which  the 
earnest  microscopist  must  teach  himself  by  observation 
and  by  experience. 

A  change  of  eye-pieces  will  likewise  call  for  another 
adjustment  of  the  objective.  Taking  out  the  two-inch, 
or  A,  ocular  and  dropping  in  the  B,  or  higher-power 
eye-piece,  will  make  a  change  for  the  worse  in  the  im- 
age obtained  with  the  best  objective.  And  in  this  case, 
to  bring  back  the  perfection  so  much  prized  by  the  ac- 
complished manipulator,  will  call  for  a  movement  of 
the  adjustment-collar  further  toward  the  open  point,  or 
zero,  the  reason  being  that  with  the  shorter  (higher- 
power)  oculars  the  optical  tube-length  is  increased,  the 
effect  being  similar  to  that  obtainable  by  the  use  of  a 


150  MICROSCOPICAL    PRAXIS. 

cover-glass  thinner  than  that  for  which  a  non-adjustable 
objective  has  been  corrected,  and  calling  for  the  ex- 
tension of  the  draw-tube,  or  for  an  adjustment  of  the 
collar  further  toward  the  open  point.  A  return  to  the 
use  of  the  lower-power  eye-piece  will  again  produce  a 
deterioration  in  the  image,  and  will  call  for  the  shorten- 
ing of  the  draw-tube,  or  the  replacing  of  the  adjust- 
ment-collar at  the  point  toward  "closed"  at  which  it 
was  before  the  higher-power  ocular  was  employed. 

This  holds  true  with  the  ordinary  Huyghenian  oculajs, 
but  with  the  compensating  eye-pieces  as  made  by  Zeiss, 
the  microscopist  is  relieved  of  this  necessity  since  these 
splendid  optical  tools  are  so  constructed  that  the  focal 
plane  is  at  the  same  place  in  each,  the  image  being 
formed  in  the  same  relative  position  with  them  all. 
This,  aside  from  the  improvement  which  they  produce 
with  even  the  ordinary  high-power  objectives,  is  an- 
other argument  for  their  use;  but  the  reader  should  re- 
member that,  while  the  ordinary  achromatic  object- 
tives  are  improved  in  performance  by  the  use  of  Zeiss's 
compensating  eye-pieces,  it  is  only  the.  high  powers 
that  are  so  improved.  The  lower  powers  are  affected 
for  the  worse.  It  is  therefore  to  the  advantage  of  the 
microscopist  that  wishes  to  work  with  high  and 
low  powers,  to  have  within  his  reach  both  kinds  of  eye- 
pieces, provided  he  desires  to  get  the  best  images 
which  all  his  objectives  are  capable  of  producing. 

While  the  compensating  oculars  of  Abbe  and  Ziess 
were  designed  primarily  to  cancel  the  slight  amount  of 
chromatic  aberration  remaining  in  the  apochromatic 
objectives,  the  permanent  position  of  their  focal  plane 
was  a  secondary  consideration,  but  an  exceedingly  use- 
ful one,  as  by  their  employment  a  change  in  the  focus 
of  the  optical  combination  is  rendered  unnecessary. 


MICROSCOPICAL    PRAXIS.  151 

When  a  higher-power  compensating  eye-piece  is  substi- 
tuted for  a  lower  power,  no  change  is  necessary  in  the 
position  of  the  objective's  collar-adjustment. 

Some  time  ago  Mr.  Edward  Pennock,  an  accom- 
plished theoretical  optician  and  expert  manipulator  of 
objectives,  suggested  the  construction  of  a  series  of 
oculars  that  should  have  one  of  the  good  qualities  pos- 
sessed by  the  compensating,  although  imitation  was 
not  thought  of,  my  impression  being  that  these  special 
"parfocal"  oculars  were  suggested  and  made  before 
the  compensating  eye-pieces  were  put  in  the  market. 

The  Latin  name,  parfocal  or  equal  focus,  explains 
the  special  quality  of  these  productions,  the  one  in 
which  they  resemble  the  compensating.  Such  oculars 
are  made  by  Messrs.  Queen  &  Co.,  of  Philadelphia,  and 
by  the  Bausch  &  Lomb  Optical  Co.,  of  Rochester,  N. 
Y.  It  would  be  an  improvement  if  all  opticians  would 
adopt  the  principle,  as  the  microscopist  would  then  gain 
an  important  convenience,  and  as  they  do  not  require  a 
different  collar-adjustment  of  the  objective  with  a 
change  from  one  magnifying  power  to  another,  the  ad- 
vantage would  be  great  indeed. 

I  have  never  had  the  pleasure  of  examining  parfocal 
eye-pieces  with  this  subject  of  collar-adjustment  in  mind, 
but  I  am  assured  by  Mr.  Edward  Pennock  that  no  re- 
adjustment should  be  required,  and  by  the  Bausch  & 
Lomb  Optical  Company  that  with  their  eye-pieces 
"when  substituting  a  high  power  for  a  low  power,  it  is 
not  necessary  to  change  the  collar-correction  of  the  ob- 
jective; a  slight  turn  of  the  micrometer-screw  (fine-ad- 
justment screw)  is  all  that  is  necessary." 

Thus  the  microscopist  that  has  the  privilege  of  using 
the  parfocal  oculars,  and  has  carefully  adjusted  his  ob- 
jective under  another  power  than  that  which  he  desires 


152  MICROSCOPICAL    PRAXIS. 

finally  to  use,  may  make  the  change  with  impunity, 
feeling  sure  that  if  the  correction  has  been  carefully 
made,  he  is  losing  nothing  in  the  good  performance  of 
the  lens,  although  he  may  not  have  exercised  his  eye 
over  the  performance  of  the  objective  with  that  special 
power  of  ocular.  This  is  an  exceedingly  great  advan- 
tage in  favor  of  the  parfocals,  and  one  that  should 
bring  them  into  more  general  use. 

There  are  few  exercises  in  the  educating  of  the 
microscopist's  eye  better  adapted  to  teach  him  how  to 
obtain  the  best  which  his  objective  can  give,  than  this 
experimenting  with  the  effect  on  the  position  of  the  ad- 
justment-collar after  a  change  of  Huyghenian  eye- 
pieces over  wide-angled  objectives.  An  eye  sensitive 
to  the  usually  slight  deterioration  in  the  image  may  be 
considered  a  valuable  possession  by  its  owner,  since 
with  it  he  will  be  able  to  distinguish  smaller  objects 
and  finer  details,  than  will  the  microscopist  that  has 
neglected  to  teach  himself  this  accomplishment,  and  to 
train  his  eye  to  be  the  valuable  servant  it  should  be, 
and  which  it  will  be  if  its  responsible  owner  will  do  his 
part. 

Balsam-mounted  diatoms  are  of  course  the  best  ob- 
jects over  which  to  make  the  experiments,  and  the 
secondary  structure  of  those  diatoms  is  perhaps  the 
most  valuable  object  which  may  be  used  as  the  test. 
Even  with  the  best  of  wide-angled  objectives  it  is  no 
light  task  to  see  distinctly  a  fracture  passing  through 
this  minute  secondary  structure,  yet  it  can  be  done, 
after  the  necessary  preliminary  eye-training,  a  training 
which  will  amply  repay,  in  future  service,  the  outlay  of 
time,  patience  and  labor  demanded  in  its  attainment. 
If  the  microscopist  is  to  enter  into  the  serious  investi- 
gation of  any  natural  object,  either  for  his  amusement 


MICROSCOPICAL    PRAXIS.  153 

only  or  for  the  instruction  of  his  fellows,  he  should  con- 
sider the  time  well  spent  which  he  uses  in  the  most 
careful  study  of  the  action  of  his  objectives  with  vari- 
ous immersion-fluids,  and  under  various  eye-pieces,  and 
with  as  many  kinds  of  secondary  diatom-structure  as 
he  may  be  able  to  obtain.  The  more  acute  the  eye 
becomes  in  this  work,  for  it  is  work  of  the  most  deli- 
cate character,  the  surer  he  may  be  that  he  is  getting 
the  best  out  of  his  objectives  when  he  comes  to  use 
them  over  an  object  about  whose  structure  he  is 
ignorant,  and  the  surer  too  that  he  is  getting  a  true 
image. 

To  the  diatoms  the  world  owes  a  debt  of  gratitude 
which  it  can  never  pay,  for  it  is  to  the  study  of  them 
that  is  due  in  great  part  the  immense  advances  made 
in  the  construction  of  the  modern  objective,  and  to  this 
the  world  owes  more  than  perhaps  it  knows.  To  sup- 
ply the  amateur  microscopist  with  the^objectives  that 
he  has  demanded  so  that  he  may  be  able  to  see  a  few 
more  striae  on  a  certain  diatom,  or  to  see  them  better, 
the  optician  has  exerted  himself  to  apply  all  his  optical 
knowledge  and  to  seek  more,  so  that  he  may  respond 
to  the  demands  coming  primarily  from  the  diatom  but 
through  the  amateur  microscojjist.  Without  the  ama- 
teur microscopist  and  his  efforts  to  have  his  objectives 
so  improved  that  he  might  the  better  resolve  his  little 
diatom,  the  microscopical  world  would  never  have  had 
the  immersion-objective  of  the  present  day,  and  the 
great  world  of  medical  science  would  never  have  known 
of  the  cholera  spirillum  nor  of  the  Bacillus  tuberculosis. 

It  is  not  too  much  to  predict  that  within  only  a  few 
years  the  cholera  will  have  become  a  disease  unknown 
except  as  it  is  remembered  by  the  aged  physician,  or  is 
recorded  in  the  books.  Having  learned  the  micro- 


154  MICROSCOPICAL    PRAXIS. 

scopic  cause,  the  scientific  man  will  before  long  be  able 
to  overcome  it,  and  to  stamp  out  of  existance  the  terri- 
ble disease  which  it  has  provoked.  If  the  microscope 
had  never  done  anything  more  than  this  it  would  de- 
serve an  immortal  fame.  But  it  has  done  more,  and 
will  do  still  more  in  the  future  as  the  amateur  micros- 
copist  shall  continue,  as  he  will  continue,  to  stimulate 
the  opticians  to  renewed  efforts  to  give  the  advanced 
worker  new  and  better  tools  with  which  to  prosecute 
his  investigations.  Without  the  amateur,  the  micro- 
scope of  to-day  would  be  only  a  dream  of  the  future; 
without  the  diatom  and  the  fussing  of  the  amateur  over 
the  striations  of  that  diatom,  the  microscopical  world 
would  still- be  lingering  near  the  edge  of  the  dark  ages, 
and  histological  and  hygienic  science  would  yet  be  in 
their  swaddling  clothes. 

But  the  diatom  and  its  good  work  have  not  yet  been 
exhausted.  It  is  only  within  a  short  time  that  the  mi- 
croscopist  has  suspected  that  it  has  a  secondary  struc- 
ture a  thousand  times  more  delicate  than  are  the  striae 
over  which  he  has  labored  so  long  and  so  hard;  and  it  is 
still  more  recently  that  he  has  suspected,  from  the  little 
that  he  has  thus  far  been  able  to  see,  that  there  is  within 
the  secondary  a  tertiary*  structure,  after  which  he  is  to 
strive,  and  for  which  the  optician  should  begin  to  pre- 
pare himself.  And  all  this  reacts  for  the  better,  and 
brings  about  the  "higher  criticism"  in  the  best  sense, 
for  while  the  scientific  world  is  yet  tempted  to  smile  at 
the  enthusiasm  of  the  amateur  microscopist  it  never  re- 
fuses to  take  advantage  of  what  that  amateur  offers. 

The  microscopist  then  that  trains  his  eye,  and  studies 
the  action  of  his  objective  under  various  conditions,  is 
not  wasting  his  time.  The  more  he  can  see  of  the  in- 
exhaustable  microscopic  world  and  the  better  he  can 


MICROSCOPICAL    PRAXIS.  155 

see  it,  the  happier  will  he  himself  be,  the  better  in- 
formed will  be  his  successor  and  the  better  prepared  to 
take  up  the  work  when  his  predecessor  shall  have  fal- 
len on  sleep. 

To  ascertain  the  initial  power  of  the  objective,  that 
is,  the  power  without  the  eye-piece,  make  the  body-tube 
about  ten  inches  long  from  the  back  lens,  focus  the  mi- 
croscope on  a  stage-micrometer  ruled  to  hundredths  of 
an  inch  for  use  with  powers  up  to  the  one-fourth  or  the 
one-fifth,  to  thousandths  of  an  inch  for  higher  powers, 
and  place  the  instrument  in  a  horizontal  position.  Re- 
move the  ocular,  and  over  the  body-tube  fasten  a  piece 
of  translucent  paper,  or  a  slip  of  finely-ground  glass, 
and  if  necessary  re-focus  until  the  micrometer-lines  are 
distinctly  seen  on  the  paper  when  strong  light  is  re- 
flected up  the  tube.  It  is  usually  necessary  to  use  di- 
rect sunlight,  although  strong  lamp-light  will  some- 
times answer  the  purpose.  Mark  the  lines  on  the 
paper,  and  measure  the  spaces  with  a  rule  divided  to 
tenths  of  an  inch.  If  the  micrometer  one-hundredth 
inch  spaces  become  one-tenth  inch  on  the  paper,  the  am- 
plification is  represented  by  ten;  if  five-tenths,  fifty,  each 
tenth  representing  an  amplification  of  ten.  If  the  one- 
thousandth  inch  spaces  on  the  micrometer  are  used 
with  high-powers,  as  they  must  be,  and  this  space  be- 
comes on  the  paper  one-tenth  inch,  then  each  tenth  will 
represent  an  amplification  of  one  hundred. 

Mr.  E.  M.  Nelson  has  suggested  another  method 
which  may  be  more  accurate,  but  which  is  scarcely  as 
convenient.  He  describes  it  as  follows: 

In  practically  measuring  the  power,  it  will  be  found  a 
more  accurate  plan  to  increase  the  distance  [that  is, 
from  the  objective  to  a  screen,  or  to  the  paper  on  the 
end  of  the  draw-tube],  to,  say,  60  inches,  and  to  di- 


156  MICROSCOPICAL    PRAXIS. 

vide  the  result  by  six.  These  measurements  are  very 
easily  performed  when  one  has  a  camera,  but  it  is  not 
so  easy  to  do  them  without.  Therefore,  another  and 
somewhat"  loose  way  of  getting  at  the  initial  power, 
that  is,  the  power  of  the  objective  alone,  is  as  follows: 
Measure  the  combined  magnifying  power  of  the  object- 
ive and,  say,  the  two-inch,  or  A^  eye-piece,  and  divide 
the  result  by  5.  This  method  would  do  very  well  if 
the  multiplying  power  of  the  eye-piece  was  5,  and  if  the 
length  of  the  body  remained  constant.  As  it  is  not  an 
easy  matter  to  find  out  the  exact  multiplying  power  of 
an  eye-piece,  Mr.  Nelson  recommends  any  one  desir- 
ous of  knowing  this  to  measure,  or  to  get  measured, 
the  initial  power  of  one  of  his  objectives;  then  measure 
the  combined  power  of  this  lens  and  the  eye-piece,  pay- 
ing great  attention  to  the  tube-length  during  the  oper- 
ation. This  will  give  him  once  for  all  the  multiplying 
power  of  his  eye-piece  with  that  tube-length.  He 
will  then  be  in  a  position  to  ascertain  the  initial  power 
of  any  other  lens  with  that  eye-piece  and  the  same  tube- 
length.  But  as  the  optical  tube-length  may  differ  from 
the  actual  tube-length,  and  does  differ  to  a  certain  ex- 
tent with  objectives  of  ordinary  construction,  the  pro- 
cess is  not  so  simple  as  it  seems.  In  order  to  get  fairly 
accurate  results  with  the  higher  powers,  a  certain  per- 
centage must  be  deducted.  To  give  some  examples:— 
Thus  the  one-inch  objective  at  60  inches  from  a 
screen  increases  the  image  of  o.oi  inch  to  0.66  inch,  its 
power  therefore  is  66,  which  at  10  inches  becomes  n, 
or  the  initial  power.  The  combined  power  of  this  lens 
with  the  A  eye-piece  is  55,  which  gives  5  as  the  multiply- 
ing power  of  the  eye-piece.  Now  if  the  combined  power 
of  this  eye-piece  with  a^  inch  objective  is  75,  we  may 
assume  that  the  initial  power  of  the  YI>  is  15. 


MICROSCOPICAL    PRAXIS.  157 

If  however,  we  treat  higher  powers  in  the  same  way, 
we  shall  get  too  high  values.  Thus  the  combined 
power  of  a  ^  and  the  eye-piece  is  203;  dividing  by  5  we 
get  40.6  as  the  initial  power  whereas  39.3  is  the  real 
power. 

Again,  the  combined  power  of  a  certain  -fa  and  the 
eye-piece  is  600,  which  divided  by  5  gives  120  as  the 
initial  power,  whereas  it  is  in  reality  113.2.  The 
empirical  rule  employed  by  Mr.  Nelson  is  to  deduct  2 
per  cent,  for  the  ^  inch  objective;  3  per  cent,  for  the 
% ;  4  per  cent,  for  the  -J-;  6  per  cent,  for  the  -J-,  the  fa,  etc. 
Thus,  taking  the  y±  mentioned  and  deducting  3  per 
cent,  from  the  203  we  get  197,  which  divided  by  5  gives 
39.4,  a  result  very  near  the  truth. 

A  certain  -J  gives  a  combined  power  of  450;  deduct 
6  per  cent,  and  we  have  423;  dividing  by  5  gives  84.6, 
the  actual  power  being  85.  For  short  bodies  of  6^  in- 
ches in  length,  or  the  Continental  size,  a  different  per- 
centage must  be  employed.  The  following  gives  fair 
results:  2  per  cent  for  y2\  4  per  cent  for  ^;  6  per  cent 
for  |-,  8  per  cent  for  -J-;  and  10  per  cent  for  fa. 

In  reference  again  to  magnifying  power,  Mr.  E.  M. 
Nelson  says  in  another  and  more  recent  paper,  that  the 
limit  of  combined  power  for  best  definition  maybe  found 
for  any  objective  of  any  given  aperture,  by  multiplying 
its  N.  A',  by  400,  stating  as  an  example,  that  the  limit 
of  power  for  best  definition  with  a  z/z  inch  of  0.3  N.  A. 
is  1 20  diameters. 

If,  in  this  case,  the  ^  inch  is  truly  named  it  will  have 
the  initial  power  of  15  diameters.  To  ac£rtain  the 
highest  powers  that  may  be  applied  in  the  eye-piece  to 
be  used,  divide  the  120  diameters  by  the  15  and  the  re- 
sult, 8,  will  be  the  required  amplifying  power  of  the 
occular  needed  to  work  the  objective  up  to  its»  limit  of 


158  MICROSCOPICAL    PRAXIS. 

good  definition,  according  to  Mr.  Nelson's  theory,  which 
is  doubtless  correct. 


To  Measure  the   Focal  Length  of  an  Objective. 

To  ascertain  this  for  an  objective  whose  box  or 
mounting  has  not  been  marked  by  the  maker,  a  rare 
occurrence,  or  when  it  is  suspected  to  be  incorrectly 
named  or  under-rated,  a  method  is  to  multiply  the  tube- 
length  by  the  quotient  obtained  by  dividing  the  dia- 
meter of  the  object  by  that  of  the  image.  If  F  is  focal 
length,  /  the  tube-length,  D  the  diameter  of  the  image, 
and  d  that  of  the  object,  the  formula  will  be,  F—  (^)/; 
and  if  the  stage-micrometer  is  ruled  to  hundredths  of  an 
inch,  the  body-tube  ten  inches  long,  and  one  micrometer- 
space  used  as  the  object  becomes  five  tenths  inch  on 
the  paper  at  the  top  of  the  tube,  then  J?=(?±*)IO  Or| 
inch. 


MICROSCOPICAL    PRAXIS.  159 

High  Magnifying  Power. 

The  highest-power  objectives  ever  made  are  a  one- 
seventy-fifth  by  the  late  R.  B.  Tolles,  of  Boston,  and  a 
one-eightieth  by  Powell  and  Lealand,  of  London.  The 
immense  amplification  obtainable  by  such  objectives  is 
chiefly  a  curiosity.  It  has  been  said  that  anything 
more  miserable  than  the  one-seventy-fifth  need  not  be 
desired,  a  statement  that  I  can  readily  believe.  No 
discoveries  have  ever  been  reported  as  having  been 
made  with  it,  so  far  as  I  know,  and  few  observations 
have  been  described.  In  reference  to  the  Powell  and 
Lealand  one-eightieth  I  have  heard  nothing. 

The  use  of  such  objectives  calls  for  the  highest  skill 
of  the  microscopist,  and  to  see  anything  with  them  de- 
mands more  than  ingenuity.  No  cover-glass  can  be  had 
thin  enough  to  allow  the  -fa  to  act  through  it;  mica 
films  are  necessary. 

A  mistake  often  made  by  the  amateur  is  to  use  too 
high  a  magnifying  power.  Mere  size  in  the  image  is 
not  a  necessary  nor  even  a  useful  feature.  It  is  rather 
a  detriment.  The  microscopist  should  use  the  lowest 
magnifying  power  compatable  with  distinctness  of  the 
image,  and  with  the  requisite  separation  of  minute  and 
contiguous  parts.  The  image  is  best  when  it  is  clear 
and  vivid,  rather  than  huge  and  foggy.  It  is  separation 
of  closety  contiguous  parts,  not  the  magnitude  of  those 
parts,  that  the  eye  needs  and  appreciates. 

It  is  probable  that  2,000  diameters  represent  the 
limit  of  useful  amplification.  Very  much  more  has 
been  used  but  not  with  praiseworthy  results. 


l6o  MICROSCOPICAL    PRAXIS. 

Immersion-objectives. 

Immersion-objectives  are  those  that  require  a  drop  of 
liquid  between  the  front  lens  and  the  cover-glass  when 
in  use.  Among  the  advantages  obtainable  by  their  em- 
ployment are  increase  of  working-distance  and  increase 
of  numerical  aperture,  with,  as  a  consequence,  the  ad- 
mission of  more  light,  and  especially  the  partial,  or  in 
some  cases,  the  complete  extinguishment  of  the  cover- 
glass,  by  which  its  aberrations  are  obviated  and  two  re- 
flecting surfaces,  those  of  the  cover  and  of  the  front 
lens,  are  cancelled.  The  honor  of  discovering  the  prin- 
ciple is  usually  conceded  to  the  renowned  Italian  pro- 
fessor, G.  B.  Amici,  who  exhibited  water-immersion  ob- 
jectives at  Paris  in  1855.  It  is  also  said  that  he  used 
oil  as  well  as  water  for  the  immersion-medium,  and  that 
he  therefore  deserves  the  credit  for  originating  oil-im- 
mersion objectives.  For  the  modern  homogeneous-im- 
mersion, however,  we  are  indebted  to  Mr.  J.  W.  Steven- 
son, a  well-known  British  microscopist.  It  was  he  who 
suggested  to  Professor  Abbe  and  to  Dr.  Zeiss  that  they 
should  turn  their  attention  to  the  theoretical  and  the 
practical  application  of  the  principle;  it  is  to  him,  there- 
fore, that  we  owe  this  great  modern  advance  in  practi- 
cal microscopy. 

The  use  of  immersion-objectives  is  attended  by  a  lit- 
tle more  inconvenience  than  that  of  dry  objectives,  and 
the  immersion  fluid  is  likely  to  be  carried  over  the  edge 
of  the  cover  by  the  movements  of  the  stage,  and  so  is 
liable  to  mingle  with  the  mounting  medium  in  those 
preparations  which  are  not  permanently  sealed.  This 
annoyance  may  be  avoided  to  a  great  extent  by  using 
square  covers  somewhat  larger  than  the  cement  ring 
enclosing  the  object,  the  movements  of  the  stage  then 
bringing  the  ring  indistinctly  into  view,  and  the  pro- 


MICROSCOPICAL    PRAXIS.  l6l 

jecting  borders  of  the  square  protecting  both  the  im- 
mersion-fluid above  it,  and  the  object  beneath. 

The  front  lens  of  these 'objectives  must  also  be  care- 
fully cleaned  and  dried  after  the  immersion-fluid  has 
been  as  carefully  applied.  But  the  advantages  obtain- 
able more  than  counterbalance  the  inconveniences. 

Tlie  fluid  is  always  applied  between  the  lens  and  the 
cover-glass.  It  is  difficult  to  imagine  any  human  being 
endowed  with  such  unmitigated  stupidity,  that  he  should 
pour  the  immersion-liquid  into  the  tube  of  the  lens- 
mounting,  yet  instances  of  the  kind  have  been  reported. 

It  is  recommended  by  some  that  the  water,  glycerine, 
oil  or  other  fluid  be  applied  in  a  small  drop  to  the  cover, 
and  the  objective  racked  down  until  the  front  lens 
comes  in  contact  with  it.  The  only  advantage  of  this 
method,  and  that  advantage  is  very  slight,  is  that  by  it 
the  probability  of  disarranging  the  object  by  the  pres- 
sure of  the  thick  liquid  compressed  between  the  cover 
and  the  lens,  is  lessened;  and  the  reader  may  prefer  this 
method,  especially  when  using  glycerine-immersion  or 
homogeneous-immersion  objectives,  but  a  great  dis- 
advantage is  that  the  moment  the  objective  touches 
the  liquid,  the  microscopist  loses  the  power  to  appreci- 
ate the  distance  between  the  lens  and  the  cover,  and  is 
therefore  likely  to  rack  down  too  far  and  so  to  do  some 
damage,  or  not  far  enough  and  thus  leave  too  much  to 
be  done  by  the  fine-adjustment  screw. 

When  using  glycerine  or  homogeneous-fluid,  I  am  in 
the  habit  of  applying  a  drop  to  the  front  lens  of  the  objec- 
tive instead  of  to  the  cover-glass.  This  may  be  done  by 
means  of  the  cork  from  the  bottle  of  fluid,  a  drop 
being  allowed  to  form  at  one  edge,  whence  it  is  care- 
fully placed  on  the  objective  without  touching  the  cork 
to  the  lens-front;  or  a  rod  may  be  forced  through  the 

12 


I  62  MICROSCOPICAL    PRAXIS. 

cork  and  the  drop  adhering  to  this  applied  to  the  lens. 
One  leg  of  a  rubber  hair-pin  forced  through  the  cork  is 
useful  for  the  purpose,  as  none  of  the  chemical  liquids 
used  for  immersion  purposes  will  act  on  it,  as  some  of 
them  will  act  on  a  metal  wire.  When  the  drop  has  been 
applied  to  the  front  of  the  lens,  the  objective  is  attached 
to  the  body-tube  and  racked  down  until  the  fluid 
touches  the  cover;  and  as  the  microscopist  looks  across 
the  slide,  between  the  cover  and  the  lens,  the  objective 
is  still  further  lowered  while  the  lessening  distance  and 
the  expansion  of  the  fluid  are  watched,  the  expansion 
being  continued  until  the  objective  is  supposed  to  be 
approximately  focussed,  when  the  fine-adjustment  fo- 
cusses  it  upward  or  downward  to  the  proper  point. 
These  movements  demand  exceedingly  great  delibera- 
tion and  caution;  deliberation  so  that  the  object  may 
not  be  disarranged  by  the  sjow  expansion  of  the  thick 
immersion-fluid,  if  it  is  not  permanently  -mounted,  and 
caution  that  the  objective  be  not  injured,  for  immersion 
lenses  are  easily  disordered.  Their  lenses  are  larger 
than  those  of  smaller  angled  dry  objectives,  their  con- 
struction is  more  delicate,  and  they  must  be  treated 
with  more  care. 

When  a  water-immersion  is  to  be  used,  that  is,  an  ob- 
jective with  which  water  is  the  immersion  liquid,  I  am 
accustomed  to  focus  it  as  a  dry  objective,  as  may  easily 
be  done,  although  the  definition  will  probably  be  abomi- 
nable and  the  field  -dimly  lighted,  yet  enough  may  be 
seen  to  show  that  the  desired  object  is  in  view.  With 
a  camel's-hair  brush  a  drop  of  water  is  then  added  to 
the  cover  near  the  edge  of  the  objective,  under  which 
it  will  run  by  capillary  attraction.  Here  all  danger  of 
forcing  the  object  out  of  position  or  of  injuring  the  ob- 
jective or  the  cover,  is  with  ordinary  caution,  reduced 
to  nothing. 


MICROSCOPICAL    PRAXIS.  163 

To  clean  the  front  of  a  water-immersion,  after  using 
it,  the  careful  employment  of  the  Japanese  filter-paper 
is  all  that  is  needed.  To  remove  the  glycerine  when 
used  by  itself  or  in  combination  with  a  salt,  as  in  the 
homogeneous-immersion  fluids,  I  am  accustomed  to 
wipe  away  the  greater  portion  with  the  Japanese  paper, 
and  to  remove  the  rest  with  a  few  touches. of  the 
tongue,  finishing  with  the  dry  paper. 

The  cedar-oil  used  with  homogeneous-immersion 
lenses  is  thickened  with  dammar,  so  that  a  touch  of  the 
moist  tongue  is  likely  to  cause  a  deposit  of  some  of 
the  gum  on  the  lens,  and  to  necessitate  repeated  appli- 
cations of  the  paper  moistened  with  alcohol  to  remove 
it.*  It  is  better  with  this  liquid  to  employ  the  paper 
alone,  and  to  finish  with  another  piece  moistened  with 
alcohol,  and  to  wipe  the  lens  dry  and  perfectly  clean 
with  still  another  piece. 

It  is  always  well  to  clean  an  immersion-objective  as 
soon  as  possible  after  using  it.  I  have  known  instances 
in  which  the  front  lens  was  so  insecurely  burnished  into 
the  metal  cell  that  the  fluid  has  penetrated  to  the  back 
of  the  glass,  and  made  necessary  a  journey  to  the  manu- 
facturer. When  the  objective  is  to  be  out  of  use  for 
only  a  short  time,  it  should  be  placed  on  the  table  with 
the  glass  surface  upward,  and  when  the  evening's  work 
is  finished,  it  should  be  carefully  returned  to  its  brass 
box,  after  a  scrupulously  neat  cleaning. 

When  about  to  measure  the  angular  aperture  of  an 
immersion-objective  the  front  must  of  course  be  im- 
mersed in  the  proper  medium.  This  may  be  done  by 
applying  a  thin  cover-glass  to  the  front  lens  by  means 
of  a  drop  of  its  special  fluid.  It  is  better  however,  to 
estimate  the  numerical  aperture  by  the  method  already 
described. 

*  Since  this  was  written  I  have  received  from  the  Bausch  &  Lomb  Optical  Com- 
pany, a  supply  of  cedar-oil  not  open  to  this  objection. 


164  MICROSCOPICAL    PRAXIS. 


To   Measure   the   Refractive   Index  of  the   Im- 
mersion-fluid. 

The  kinds  of  homogeneous-immersion  media  are 
numerous,  the  object  of  all  being  to  reach  a  refractive 
index  as  nearly  as  possible  that  of  the  front  lens  of  the 
objective.  The  basis  is  generally  pure  glycerine  in 
which  is  dissolved,  usually  by  the  aid  of  heat,  various 
-chemical  salts,  among  which  are  chloral  hydrate,  zinc 
sulpho-carbolate,  cadmium  chloride,  zinc  iodide,  dis- 
tilled zinc-chloride,  and  perhaps  others.  The  micros- 
•copist  himself  may  prepare  any  of  these  fluids,  but  he 
"would  better  buy  them  of  the  optician  that  makes  his 
objectives.  He  will  thus  not  only  save  himself  trouble, 
but  he  will  be  sure  that  the  index  of  the  fluid  will  be 
as  nearly  correct  as  may  be.  Upon  the  proper  condi- 
tion of  the  immersion-fluid  depends  the  proper  action 
of  the  objective,  the  best  performance  of  the  best  lenses 
being  defeated  by  a  liquid  that  is  not  of  the  correct  re- 
fractive index. 

In  any  event  it  is  important  that  the  medium  should 
have  a  refractive  index  as  nearly  like  that  of  liquid 
crown-glass  as  possible,  and  for  the  purpose  of  learning 
its  condition  in  this  respect,  several  devices  have  been 
suggested.  With  his  cedar-oil,  Zeiss  sends  out  a  bottle 
with  flattened  sides,  to  the  stopper  of  which  is  attached 
a  glass  wedge  which  is  to  be  immersed  in  the  medium 
and  held  against  the  light,  when  certain  appearances  in 
a  distant  object  will  show  when  the  correct  refractive 
index  has  been  attained.  Not  long  ago  Prof.  Hamilton 


MICROSCOPICAL    PRAXIS. 


165 


Fig.  it.     H.  L.  Smith's  test-slide 
for  refractive  index. 


L.  Smith,  the  well-known  American  microscopist, 
devised  a  little  instru- 
ment which  is  more  con- 
venient and  more  accur- 
ate than  Zeiss's.  It  is 
shown  in  Fig.  n,  and  is 
described  by  its  inventor 
as  follows. 

It  consists  of  a  brass 
adapter  with  the  society  screw  above  it  to  attach  it  to 
the  body-tube  of  the  microscope,  and  one  below  it  to 
receive  an  objective,  the  one-inch  being  generally  used 
with  it.  Through  openings  in  the  sides  two  glass  slips 
are  inserted,  one  having  a  polished  cavity  near  the  end. 
These  slips  are  of  crown-glass  with  a  refractive  index 
as  nearly  as  possible  that  of  the  cover-glass.  To  test 
the  medium,  a  drop  is  put  in  the  concavity,  the  slips 
placed  together,  and  inserted  in  the  adapter  above  the 
objective.  The  medium  passes  between  the  two  by 
capillary  attraction,  and  the  microscope  is  focussed  on 
an  object,  the  microscopist  looking  through  the  slips 
and  the  interposed  medium.  The  focus  will  not  differ 
with  or  without  the  glass  slips,  and  when  the  concavity 
is  pushed  directly  over  the  objective,  if  the  medium  be 
optically  homogeneous  with  the  slips,  the  focus  will 
need  no  change  and  the  definition  will  be  unimpaired. 
But  if  the  medium  has  not  the  proper  index,  white  the 
focus  may  need  no  alteration,  the  outlines  of  the  object 
will  be  surrounded  with  colored  fringes. 

If  the  focus  has  been  obtained  by  means  of  the  rack 
and  pinion,  the  fine-adjustment  always  remaining  the 
same,  one  can  readily  ascertain,  in  the  following  way, 
the  refractive  indices  of  the  various  media  proposed  for 
use  with  immersion-objectives.  Let  a  mark  be  placed 


l66  MICROSCOPICAL    PRAXIS. 

on  the  rack-bar  or  on  the  sliding  draw-tube,  as  the  case 
may  be,  when  the  focus  is  obtained  with  the  glass  slips 
in  the  position  shown  in  the  figure  (Fig.  n);  this  mark 
will  indicate,  for  example,  a  refractive  index  of  1.52,  or 
that  of  crown-glass.  Filling  the  concave  now  with 
cinnamon-oil,  and  focussing  again  (using  the  same  ob- 
jective and  eye-piece),  we  get  another  position  for  a 
mark  indicating  a  refractive  index  of  1.6,  the  index  of 
cinnamon-oil.  Using  water  we  get  still  another,  1.33; 
and  glycerine  1.41;  the  extremes  will  be  about  half  an 
inch  apart,  as  measured  by  the  bar  or  by  the  tube,  and 
by  interpolation,  we  can  thus  get  pretty  nearly  the  re- 
fractive index  of  any  fluid  medium.  Prof.  Smith  re- 
ports that  he  has  found  the  so-called  homogeneous 
media  sold  in  the  shops  to  differ  very  greatly,  fully  one- 
fourth  of  an  inch  out  of  the  way  in  many  cases. 

When  one  has  a  fine  objective  and  with  a  certain 
immersion-fluid  has  obtained  certain  positions  of  the 
adjustment-collar  for  the  best  work  on  certain  tests, 
the  exact  refractive  index  of  the  medium  can  be  ascer- 
tained by  this  instrument,  and  afterwards  always 
secured.  A  non-adjustable  immersion-objective,  a  one- 
eighth  by  Spencer,  performed  most  admirably, 
both  with  oblique  and  with  direct  light,  using  the 
medium  furnished  by  the  maker,  but  showed  indiffer- 
ently well  with  another  medium  which,  on  being  tested 
with  this  little  apparatus,  required  an  alteration  of 
focus  necessary  to  obtain  distinct  vision,  or  rather  the 
most  distinct  vision,  of  fully  one-fourth  of  an  inch.  On 
diluting  the  second  medium  to  bring  it  to  the  same  in- 
dex as  that  sent  out  by  the  maker,  the  performance 
was  entirely  satisfactory.  It  will  be  understood  that 
there  should  be  a  diaphragm  in  the  adapter  of  such  a 
size  that  it  shall  prevent  the  passing  of  any  light  except 


MICROSCOPICAL    PRAXIS.  167 

what  actually  passes  through  the  fluid  when  the  con- 
cavity containing  the  immersion-fluid  to  be  tested,  is 
put  over  the  objective. 


•x- 


To  Focus  an  Objective. 

This  has  already  been  incidentally  referred  to,  but  it 
may  well  be  repeated,  as  among  his  first  lessons  the  am- 
ateur or  the  novice  should  learn  how  to  focus  his  ob- 
jectives properly,  everyone  of  which,  it  does  not  matter 
whether  it  be  a  five-inch  or  a  one-fiftieth  inch,  should 
without  exception  be  focussed  in  the  same  way.  Some- 
times there  are  two  ways  of  doing  the  same  thing. 
Here  there  is  but  one.  It  is  this:  In  no  circumstances 
should  the  objective  be  focussed  by  racking  the  body- 
tube  down  while  the  microscopist  is  looking  through 
the  instrument.  Always  look  across  the  objective,  be- 
tween the  front  lens  and  the  cover-glass,  or  the  slide, 
while  the  body-tube  is  carefully  racked  downward  until 
the  front  of  the  high-power  objective  is  almost  in  con- 
tact with  the  cover;  then,  while  the  eye  is  at  the  eye- 
piece, rack  the  body  upward  until  the  focus  is  approxi- 
mately obtained,  finishing  with  the  fine-adjustment. 
Proceed  in  this  way  always,  and  at  all  times,  and  in  all 
circumstances,  even  though  it  should  be  necessary  to 
place  your  ear  flat  on  the  table-top  whilst  you  look 
across  the  objective-front  toward  the  light,  and  it  will  be 


l68  MICROSCOPICAL    PRAXIS. 

to  the  amateur's  advantage.  It  will  be  especially  advan- 
tageous to  the  novice,  if  he  will  from  the  very  first,  cul- 
tivate the  habit  of  racking  the  body-tube  upward  when- 
ever the  slide  is  to  be  taken  from  the  stage.  This  good 
habit  will  soon  become  automatic,  and  if  both  these 
rules  be  observed,  there  will  be  no  danger  of  breaking 
or  of  scratching  the  soft  glass  forming  the  front  lens  of 
the  objective,  nor  of  injuring  the  slide.  The  latter 
might  not  be  of  any  importance,  whereas  the  breaking 
of  an  objective  is  a  disaster  that  cannot  be  remedied. 


The  Maltwood  Finder  and  Similar  Devices. 

•  Reference  has  already  been  made  to  the  difficulty  of 
finding  a  small  object,  or  some  special  part  of  a  larger 
object,  with  a  high-power  unless  a  mechanical  stage  be 
used.  The  field  of  the  high-power  objective  is  so 
small  that  the  chances  of  bringing  the  desired  object 
within  its  circumference  are  slight.  Usually  it  is  neces- 
sary to  substitute  a  low-power  lens,  bring  the  specimen 
within  its  field,  then  to  re-attach  the  higher-power  ob- 
jective, in  or  near  whose  field  the  object  should  be. 

On  mechanical  stages  there  are  commonly  engraved 
two  sets  of  lines  about  one  one-hundredth  inch  apart, 
the  scales-  resembling  those  of  a  micrometer.  These 
are  intended  to  facilitate  the  finding  of  the  object  the 
second  time,  the  objective  being  noted,  and  the  posi- 
tion of  the  stage  recorded  as  read  from  the  horizontal 
and  the  vertical  scales  on  its  surface.  When  the  stage 


MICROSCOPICAL    PRAXIS.  169 

is  again  placed  in  those  positions,  the  same  objective 
used,  and  the  slide  laid  on  the  object-carrier  as  it  was 
before,  the  object  should  then  be  in  the  field. 

Several  devices  have  been  suggested  for  the  conveni- 
ence of  those  microscopists  that  do  not  possess  a  me- 
chanical stage.  These  consist  of  lines  photographed 
ofruled  on  a  glass  slip,  their  number  usually  being 
great  and  the  spaces  between  them  small,  each  of  the 
latter  bearing  one  or  more  figures.  The  best  known  of 
these  finders  is  Maltwood's,  a  glass  plate  bearing 
twenty-five  hundred  squares,  so  numbered  that  the 
position  of  an  object  may  be  recorded  by  recording  the 
numbers  within  the  space  over  which  it  may  be  when  in 
the  field  of  a  certain  objective. 

The  object  is  brought  into  the  centre  of  the  field  of 
view,  when  the  slide  is  removed  and  the  Maltwood 
finder  substituted.  The  numbers  on  the  square  now 
occupying  the  position  previously  occupied  by  the  ob- 
ject are  noted,  and  whenever  this  special  square  is 
again  brought  into  the  field  of  that  objective,  the  stage 
will  be  in  a  position  to  bring  at  once  the  desired  object 
into  that  field. 


The  Condenser. 

The  sub-stage  condenser  is  the  most  important  ac- 
cessory that  the  microscopist  can  have,  especially  if  he 
intends  to  work  with  first-class,  wide-angled  objectives. 
Its  purpose  is  to  supply  a  wide  and  solid  cone  of  light, 


170  MICROSCOPICAL    PRAXIS. 

not  primarily  to  illuminate  the  object,  but  to  give  the 
very  light  of  life  to  the  objective  itself.  Without  a 
modern,  wide-angled  condenser  the  microscopist  is 
badly  handicapped,  although  he  may  possess  the  best 
objectives  to  be  had  for  money.  The  condenser  brings 
out  the  good  points  of  the  lenses  in  a  surprising  way, 
and  it  also  reveals  with  as  painful  distinctness,  the  bad 
qualities  of  the  same  lenses. 

The  microscopist  should  purchase  the  best  condenser 
he  can  find  at  the  opticians',  and  attach  it  permanently 
to  his  stand.  It  may  be  used  to  increase  the  illumina- 
tion until  the  eye  refuses  to  endure  it,  or  the  light,  by 
its  means,  may  be  reduced  to  the  faintest  glimmer.  By 
its  means  again,  if  it  have  the  proper  angular  aperture, 
the  whole  aperture  of  the  objective  may  be  filled  by  a 
solid  cone  of  rays;  and  by  the  use  of  the  proper  dia- 
phragms, or  by  moving  the  entire  condenser  laterally, 
illumination  of  the  greatest  obliquity  may  be  obtained 
for  the  resolving  of  tests,  or  for  the  study  of  obscure 
structures  of  a  certain  character. 

The  defining  and  the  resolving  powers  of  the  ob- 
jective are  improved  by  its  use;  indeed,  the  best,  high- 
est-power, homogeneous-immersion  lenses  will  not  do 
themselves  even  partial  justice  without  the  use  of  wide- 
angled  condensers  now  fortunately  becoming  common. 
The  apparatus  is  a  combination  of  two  or  more 
lenses  forming  an  instrument  not  unlike  a  greatly 
enlarged  objective.  It  is  fitted  to  the  sub-stage  ring 
so  that  it  may  be  accurately  centred,  a  condition  ab- 
solutely essential  to  its  best  performance,  and  so  that 
it  may  be  moved  upward  or  downward  to  bring  the 
light  from  the  mirror  to  a  focus  on  the  object,  or  to  re- 
move it  beyond  the  focus  so  as  to  reduce  the  intensity 
of  the  illumination,  although  this  method  of  using  it  is 


MICROSCOPICAL    PRAXIS.  171 

not  to  be  unreservedly  commended    with  wide-angled, 
high-power  objectives  of  the   present  day.     Some  me-' 
thod,  however,  of  changing  its  position  vertically,  either 
by  rack  and   pinion  or  by  direct  finger-movements,   is 
absolutely  necessary. 

It  is  always  accompanied  with  diaphragms  to  reduce 
the  size  of  the  illuminating  cone,  to  obtain  light  of 
great  obliquity  or  black-ground  illumination.  In  the 
best  condensers  these  diaphragms  are  applied  below 
the  lenses,  and  this  should  always  be  their  position.  In 
no  circumstances  should  they  be  above  the  front  lens 
of  the  apparatus. 

Authorities  on  the  subject  recommend  that  the  con- 
denser should  never  be  used  without  being  accurately 
focussed.  Dr.  W.  H.  Dallinger  says  that  it  should  al- 
ways be  racked  upward  until,  when  the  microscopist 
looks  down  the  body-tube  after  the  eye-piece  has  been 
removed,  the  back  lens  of  the  objective  is  three-fourths 
full  of  light.  M.  H.  Peragallo,  in  the  "Annales  de  Mi- 
crographie,"  gives  an  elaborate  discussion  of  the 
theoretical  principles  involved  in  microscopical  illum- 
ination, and  states  that  the  back  lens  of  the  objective 
should  be  only  one-third  filled  with  light.  Dr. 
Dallinger's  teaching  calls  for  more  (three-fourths),  and 
the  microscopist  that  attempts  to  use  his  objective  thus 
illuminated  will  have  an  experience  that  will  teach  him 
that,  in  ordinary  circumstances,  the  method  is  imprac- 
ticable. According  to  Dr.  Dallinger's  contention, 
which  is  here  correct,  if  the  condenser,  after  having 
been  focussed,  be  racked  a  little  further  upward  while 
the  microscopist  is  still  looking  at  the  back  lens  of  the 
objective,  there  will  soon  appear  on  each  side  of  the 
illuminated  disk  a  little  dark  spot  showing  that  the 
apparatus  has  been  raised  too  high,  and  is  within  the 


IJ2  MICROSCOPICAL    PRAXIS. 

focus,  as  the  late  Dr.  W.  H.  Carpenter  wrongly  ad- 
vised the  microscopist  to  use  it.  The  proper  position, 
according  to  Dr.  Dallinger,  is  said  to  be  that  reached 
just  before  the  dark  spots  appear;  that  is,  the  conden- 
ser is  then  focussed. 

This  is  undoubtedly  the  proper  way  in  which  to  use  the 
apparatus,  but  it  is  an  utter  impossibility  so  to  use  it, 
unless  the  microscopist  have  some  way  of  reducing  the 
terrible  intensity  of  light  which  will  then  pour  into  his 
eye.  This  reduction  is  to  be  obtained,  not  by  racking  the 
condenser  downward,  or  beyond  the  focus,  which  would 
be  using  it,  as  well  as  the  objective,  at  a  disadvantage 
if  the  objective  is  a  first-class  lens,  but  by  the  employ- 
ment of  properly  colored  glass  to  be  placed  between 
the  lamp  and  the  mirror,  or  between  the  mirror  and  the 
condenser,  the  correct  color  being  double  cobalt-blue, 
a  kind  of  glass  not  to  be  had  in  this  country,  and  until 
recently  scarce  even  in  Europe.  From  this  Dr. 
Dallinger  recommends  that  screens  be  made  and 
placed  between  the  mirror  and  the  source  of  light,  so 
to  modify  the  illumination  that  the  eye  may  endure  it. 
Or  two  disks  of  this  special  glass  may  be  prepared  and 
inserted  in  the  sub-stage  below  the  condenser,  with  the 
same  purpose  in  view. 

If  used  as  M.  Peragallo  recommends,  the  conden- 
ser must  also  be  supplied  with  the  blue  glass  to  modify 
the  illumination,  although  the  advanced  microscopist 
may  be  able  to  endure  the  unmodified  light,  since  his 
eye  has,  by  practice,  become  less  sensitive  to  such  im- 
pressions while  it  has  also  become  better  able  to  ap- 
preciate minute  points,  delicate  details  and  sharply  de- 
fined outlines.  But  while  it  may  be  possible  or  neces- 
sary to  use  only  one-third  of  the  back  lens,  or  even  less, 
doing  so  would  not  be  wording  the  objective,  nor  the 


MICROSCOPICAL    PRAXIS.  173 

condenser,  up  to  the  best,  if  condenser  and  objective 
are  first-class.  With  the  French  microscopist's  method 
we  should  be  losing  in  two  particulars,  yet  while  his 
recommendation  may  be  put  into  practice,  Dr.  Dallin- 
ger's  can  not,  unless  the  right  kind  of  blue  glass  can  be 
obtained  to  reduce  the  terrible  intensity  of  the  illumina- 
tion from  the  modern,  wide-angled  condenser.  But  the 
only  way  in  which,  at  present,  the  teaching  of  the  expert 
British  authorities  can  be  followed  is  to  send  to 
Messrs.  Powell  and  Lealand,  the  well-known  opticians 
of  London,  and  import  the  blue  glass,  as  they  are  the 
only  dealers  who,  so  far  as  I  know,  supply  the  material 
proper  for  the  purpose. 

It  is  indeed  possible  to  use  a  wide-angled  condenser 
when  almost  in  focus,  if  we  employ  a  blue  glass  chim- 
ney to  the  microscope-lamp,  and  put  below  the  con- 
denser three  thicknesses  of  the  blue  glass  as  supplied 
with  the  "Acme  lamp,"  by  Messrs.  Queen  &  Co.,  and  by 
Messrs.  James  Stratton  &  Son  with  their  "Stratton 
Illuminator."  By  this  means  a  one-tenth  inch  objec- 
tive, or  higher  power,  may  be  used  with  the  condenser 
accurately  focussed;  but  with  the  one-fifth  or  with 
lower  powers  the  light  must  still  be  modified  by  racking 
the  condenser  downward,  or  preferably  by  using  dia- 
phragms. The  unprotected  eye  can  not  endure  the 
intensity  of  the  light  under  these  conditions,  and  if 
more  blue  glass  be  inserted  into  the  sub-stage,  the  illu- 
mination becomes  too  much  weakened,  and  the  image 
is  deteriorated. 

That  the  condenser  will  give  us  its  best  service  when 
carefully  focussed  under  our  first-class  objectives,  there 
can  be  no  doubt;  and  if  the  American  microscopist  can 
modify  the  light  without  depriving  it  of  any  of  its  good 
qualities,  he  should  always  use  the  accessory  in  that 


174  MICROSCOPICAL    PRAXIS. 

position.  But  I  fear  he  will  not  be  able  to  do  so  with 
all  objectives,  unless  the  opticians  come  to  his  help,  as 
Messrs.  Powell  &  Lealand  seem  to  have  come  to  the 
help  of  the  British  microscopists,  and  give  him  the 
right  kind  of  blue  glass.  But  in  the  present  circum- 
stances, the  advantages  of  using  the  condenser  when 
accurately  focussed  are  so  great,  that  it  seems  best  to 
keep  it  in  focus  and  to  reduce  the  intensity  of  the 
illumination  by  diaphragms.  This  will  reduce  the 
angle  of  ,the  illuminating  cone,  which  is  not  always  a 
commendable  thing  to  do,  but  at  present  is  apparently 
unavoidable.  To  get  the  greatest  benefit  to  be  ob- 
tained from  a  condenser,  the  object  should  be  exactly 
at  the  apex  of  the  illuminating  cone  issuing  from  the 
front  lens  of  the  apparatus. 

The  microscopist  may  be  certain  that  the  condenser 
is  focussed  even  while  looking  through  the  instrument, 
and  without  removing  the  eye-piece  to  gaze  down  the 
body-tube.  To  do  this,  the  condenser,  after  the  ob- 
jective has  been  focussed,  is  gently  racked  upward, 
while  the  eye  is  at  the  ocular,  and  the  ascent  continued 
until  the  dust-particles  on  its  front  lens  are  visible  in 
the  field  as  blue  or  blue-bordered  objects  which  are 
rather  indistinct  yet  plainly  visible.  These  dust-parti- 
cles are  sometimes  so  conspicuous  that  they  become 
menacing  annoyances  in  observations  with  high-powers, 
and  the  front  lens  of  the  condenser  must  be  dusted  with 
a  camel's-hair  brush,  or  racked  slightly  below  the  focus. 
It  will  also  sometimes  happen  that  when  the  apparatus 
is  used  with  low  powers,  or  when  it  is  occasionally 
racked  far  downward,  that  dust-particles  and  finger- 
marks on  the  blue-glass  in  the  sub-stage  will  become 
apparent,  and  may  possibly  mislead  the  observer  into 
thinking  that  the  instrument  is  in  its  proper  position. 


MICROSCOPICAL    PRAXIS.  175 

The  widest-angled  condensers  for  use  with  the 
widest-angled  homogeneous-immersion  "objectives,  are 
so  made  that  they  may  be  employed  in  immersion  con- 
tact with  the  lower  surface  of  the  slide.  A  drop  of 
glycerine  or  of  homogeneous-immersion  fluid  is  placed 
on  the  upper  surface  of  the  condenser  and  brought  into 
contact  with  the  lower  surface  of  the  slide.  Such 
immersion-condensers  must  necessarily  be  used  in 
focus;  and  with  them  the  intensity  must  be  in  some 
way  decreased,  since  the  normal  eye  can  not  endure  it 
when  unmodified. 

There  are  two  forms  of  the  accessory  in  the  optical 
market,  one  with  no  corrections  for  the  spherical  and 
chromatic  aberrations,  the  other  carefully  corrected 
for  both  of  these  troublesome  things.  The  most  popu- 
lar form  is  Abbe's,  and  belongs  amongst  the  uncor- 
rected,  although  in  recent  years  its  accomplished  in- 
ventor has  issued  a  corrected  condenser  which  he 
recommends  for  use  in  microscopical  photography.  It 
is  also  commendable  for  ordinary,  every-day  use,  as  the 
uncorrected  form  adds  an  immense  amount  of  spherical 
aberration  for  the  objective  to  contend  against,  so  that 
diaphragms  should  always  be  used  with  it  to  cut  off 
some  of  the  uncorrected  marginal  rays. 

In  reference  to  the  uncorrected  accessory  Prof.  Abbe 
says:  "The  condenser  is  not  made  achromatic  for  the 
reason  that,  for  the  effect  contemplated,  it  would  be 
altogether  useless  to  seek  to  obtain  a  sharp  image  of 
the  cloud  or  other  source  of  light,  as  it  is  in  like 
manner  quite  immaterial  whether  the  image  is  formed 
precisely  on  a  level  with  the  object,  or  somewhat  above 
or  below  it."  Several  British  microscopists  take  issue 
with  Prof.  Abbe  on  these  points,  and  insist  that  the  con- 
denser should  be  corrected^ and  as  Prof.  Abbe  has  re- 


176  MICROSCOPICAL    PRAXIS. 

cently  made  a  corrected  form  of  his  own  celebrated 
apparatus,  it  would  seem  that  he  has  changed  his 
opinion. 

Messrs.  Powell  and  Lealand,  of  London,  make  an  oil- 
immersion  condenser  of  wide  angle  and  a  high  price; 
Watson  and  Son,  also  of  London,  make  an  excellent  con- 
denser which  is  corrected,  its  angle  being  i.o  N.  A. 
against  1.40  of  the  Powell  and  Lealand  form.  In  this 
country,  Messrs.  Bausch  and  Lomb,  of  Rochester,  N.  Y., 
make  an  achromatic  form  of  the  Abbe  condenser,  as 
well  as  another  which  is  not  corrected. 

Low-power  objectives,  those,  for  instance,  below  the 
one-fourth  or  one-fifth  inch,  and  those  of  small  angular- 
aperture,  do  not  call  for  the  use  of  the  condenser. 
Sufficient  illumination,  generally  more  than  is  needed, 
may  be  had  with  these  from  the  concave  mirror  alone. 
It  is  also  possible  to  use  an  objective  of  low-power  as  a 
condenser,  if  the  sub-stage  ring  is  supplied,  as  it  should 
be,  with  an  adapter  carrying  the  society  screw.  The 
objective,  when  used  for  this  purpose,  is  screwed  into 
the  adapter  with  the  front  lens  facing  upward,  the  light 
being  reflected  through  it  from  the  plane  mirror.  If  a 
microscopical  lamp  be  used,  or  a  bull's-eye  lens  be  in- 
terposed between-  the  lamp  and  the  mirror,  the  plane 
mirror  should  also  be  used. 

Several  diaphragms,  which  are  used  below  the  poste- 
rior lens,  for  central,  oblique  and  black-ground  illumi- 
nation, accompany  all  forms,  or  should  do  so.  Those 
for  central  light  have  a  central  opening,  the  size  of  the 
cone  of  light  and  the  obliquity  of  its  lateral  rays  vary- 
ing with  the  size  of  the  diaphragm-opening  employed. 
For  oblique  illumination  two  lune-shaped  diaphragms 
are  usually  supplied.  These  are  placed  in  the  dia- 
phragm-carrier, one  at  a  time,  of  course,  in  any  posi- 


MICROSCOPICAL    PRAXIS.  177 

tion  that  may  be  needed  to  produce  the  effects  desired; 
in  some  the  diaphragm-carrier  may  be  rotated.  For 
black-ground  illumination  those"  with  the  central  disk 
supported  by  radiating  arms  are  used,  but  to  obtain  the 
effect  with  wide-angled  objectives  something  more  is 
needed  than  the  use  of  these  special  disks.  A  circular 
diaphragm  must  also  be  placed  at  the  back  of  the  ob- 
jective. 

The  diaphragm-carrier  in  the  American  forms  of  the 
condenser  is  usually  a  sliding  plate  into  whose  aperture 
the  various  diaphragms  or  stops  are  placed,  when  it  is 
pushed  below  the  lenses  until  a  spring  catch  indicates 
that  it  is  properly  centred  to  the  condenser;  but  this 
has  nothing  to  do  with  the  centring  of  the  condenser 
to  the  objective. 

For  oblique  illumination,  the  lunate  disks  are  used, 
the  larger  when  the  greater  portion  of  the  cone  of  light 
is  to  be  intercepted,  the  smaller  when  more  of  the  rays 
nearer  the  centre  are  desired.  With  either  size  the  con- 
denser will  give  light  of  greater  obliquity  than  many  ob- 
jectives will  receive.  The  object,  however,  may  be 
obliquely  illuminated  with  rays  from  any  direction, 
either  by  withdrawing  the  carrier  and  inserting  the 
lune-shaped  diaphragm  in  another  position,  or  by  ro- 
tating it,  so  that  the  light  shall  sweep  around  a  circu- 
lar course.  This  requires  delicate  manipulation,  an  ob- 
jective of  the  proper  angular  aperture  to  receive  light 
of  that  obliquity,  and  very  accurate  centring  of  all 
the  parts.  It  may  be  done,  however,  with  fine  effect  in 
the  resolution  of  lined  objects,  diatoms  for  instance, 
but  if  the  microscopist  owns  the  wide-angled  apparatus, 
and  this  form  of  oblique  light  is  to  be  employed  with  a 
dry  objective,  the  front  hemispherical  lens  of  the  con- 
denser should  be  removed  and  the  remainder  of  the 


178  MICROSCOPICAL    PRAXIS. 

combination  focussed  on  the  object  without  a  dia- 
phragm. Then  insert  the  lunate  disk,  and,  if  all  is  well, 
a  glance  down  the  body-tube,  without  the  ocular,  will 
show  a  small  double-convex  spot  of  light  near  one 
border  of  the  back  lens  of  the  objective,  with  the  diffrac- 
tion spectra  also,  if  they  are  specially  looked  for. 

If  the  sub-stage  has  lateral  movements,  as  it  has  on 
some  first-class  stands,  oblique  illumination  with  the 
circular  diaphragm-openings  may  be  obtained,  but  some- 
what less  effectually,  by  moving  the  entire  condenser 
from  side  to  side. 

In  what  is  called  black-ground  illumination  the  ob- 
ject appears  to  be  self-luminous,  gleaming  with  the 
vivid  radiance  of  molten  silver,  seeming  to  rest  softly 
on  a  back -ground  of  the  blackest  velvet.  Living  animals 
appear  like  creatures  fashioned  from  moonbeams;  mi- 
nute particles  shimmer  and  flash  like  silver  stars;  a 
little  heap  of  colored  sand-grains  seems  a  little  heap  of 
rubies  and  diamonds  from  Sinbad  the  Sailor's  Valley  of 
Gems.  And  to  obtain  such  exquisite  pictures  it  is  only 
necessary  to  obstruct  the  central  beam  of  light  by  a  cir- 
cular, opaque  disk,  allowing  the  object  to  be  illuminated 
by  the  light  that  comes  to*  it  from  the  periphery.  No 
rays  reach  the  objective  directly.  All  must  first  enter 
the  object  and  there  be  properly  refracted  or  inflected, 
or  after  passing  through  the  object,  must  be  thrown 
back  on  it  by  reflection  from  the  cover-glass,  so  that 
under  the  beating  of  those  waves  of  light  it  shall  appear 
to  glow  with  a  soft  intensity  indescribable.  This  effect 
may  be  obtained,  sometimes  better  and  more  easily,  by 
sub-stage  apparatus  especially  intended  for  the  purpose, 
rather  than  by  any  sub-stage  condenser. 

For  black-ground  illumination,  the  central  disk-dia- 
phragm must  be  used,  and  with  the  one-inch  objective 


MICROSCOPICAL    PRAXIS.  179 

brilliant  effects  may  be  produced,  with  the  proper 
objects.  To  do  this  with  the  one-inch  of  33°  and  the 
uncorrected  Abbe  condenser  of  1.40  N.  A.,  it  is  neces- 
sary to  remove  the  front  lens  of  the  condenser,  when 
the  effect  will  be  exceedingly  fine.  Here  again  if 
the  bull's-eye  lens  is  placed  between  the  mirror  and 
the  source  of  light,  the  plane  mirror  is  to  be  used;  if 
the  light  is  taken  directly  from  the  lamp-flame,  the 
concave  mirror  is  the  proper  one  to  be  employed. 
Black-ground  illumination  may  be  obtained  with  pow- 
ers of  from  five-hundred  to  six-hundred  diameters,  but 
according  to  my  experience  it  is  not  praiseworthy. 

With  the  Abbe  form  of  the  condenser,  Zeiss  supplies 
diaphragms  to  be  applied  to  the  back  lens  of  his  wide- 
angled  objectives  when^lack-ground  illumination  is  de- 
sired with  them,  the  proper  disk-bearing  plate  being 
placed  in  the  diaphragm-carrier,  and  immersion  contact 
made  with  the  lower  surface  of  the  slide.  The  diffi- 
culty with  any  but  Zeiss's  objectives  is  to  prepare  the 
diaphragms  for  the  back  lens.  With  his  they  are  made 
of  metal,  to  fit  properly  when  dropped  into  the  mount- 
ing, and  the  opening  will  ttyen  be  central.  But  if  the 
microscopist  must  cut  them  from  paper,  he  need  not 
expect  to  obtain  the  best  results.  The  directions  are 
to  drop  the  diaphragms  into  the  back  of  the  wide-an- 
gled objective,  and  then  the  microscopist  is  left,  by  all 
opticians  except  Zeiss,  to  take  care  of  himself.  Yet  it. 
is  of  course  impossible  for  any  one  optician  to  supply 
these  little  parts,  since  no  two  objectives  by  different 
makers,  of  even  the  same  magnifying  power,  have  the- 
same  sized  mounting.  The  microscopist  must  depend 
upon  himself,  and  he  will  speedily  observe  that  when, 
the  aperture  is  reduced  by  a  diaphragm  at  the  back 
lens,  the  defining  and  resolving  powers  of  the  objective,- 


l8o  MICROSCOPICAL    PRAXIS. 

suffer  an  injurious  diminution.  The  experiment  is 
worth  making  although  no  other  result  than  this  be  ob- 
tained. 

While  black-ground  illumination  is  beautiful,  it  has 
little  scientific  value.  I  do  not  know  that  any.  discovery, 
or  even  any  observation  of  importance,  has  ever  been 
made  by  its  use.  It  will  at  times  exhibit  certain  struc- 
tural features  in  a  conspicuous  way,  but  only,  I  think, 
after  they  have  been  previously  observed,  for  in  most 
cases  this  peculiar  lighting  appears  to  make  the  struc- 
ture obscure.  It  may  render  the  contour  lines  more 
distinct,  and  develop  the  whole  object  in  a  glamour  of 
brilliant  beauty,  but  the  microscopist,  while  he  never 
disdains  beauty,  never  makes  it  the  object  of  his  pur- 
suit. 

Although  oblique  illumination  may  be  obtained  by 
the  lateral  movement  of  the  condenser,  if  the  sub-stage 
arrangements  will  allow  it,  such  use  of  the  apparatus  is 
not  to  be  commended.  It  should,  for  ordinary  work, 
when  oblique  light  is  never  wanted,  be  used  with  cen- 
tral illumination,  and  the  more  perfectly  centred  it  is 
the  better.  To  accomplish  this,  proceed  as  was  de- 
scribed in  connection  with  the  centring  of  the  illumina- 
ting beam  with  the  objective  and  the  mirror.  But  it 
will  sometimes  happen  that  while  the  condenser  ap- 
pears to  be  centred  when  it  is  near  the  focus,  that  it 
will  be  greatly  out  of  centre  when  racked  downward. 
In  such  a  common  occurrence  a  slight  change  in  the 
position  of  the  mirror,  or  of  the  lamp,  or  perhaps  of 
both,  is  all  that  will  be  needed  to  remedy  matters. 

It  frequently  happens  that  with  even  excellent  objec- 
tives, that  after  they  have  been  carefully  adjusted,  and 
the  outlines  of  the  image  are  as  narrow  and  as  black  as 
they  can  be  made,  that  a  narrow  line  of  bright  light  is 


MICROSCOPICAL    PRAXIS.  l8l 

conspicuously  visible  around  the  free  edges  of  the  ob- 
ject. This  external  line  of  light  has  been  said  to  be  an 
evidence  of  improper  adjustment,  and  that  a  second- 
rate  objective  is  being  used.  The  fact  is,  that  such  a 
bright  marginal  line  is  evidence  that  the  objective  is 
asking  for  more  light  to  be  given  it  in  a  wider  cone  of 
illumination.  The  bright  line  can  be  almost  cancelled 
in  the  image  formed  by  first-class  objectives,  and 
greatly  diminished  in  those  of  inferior  quality,  by  sup- 
plying a  wider  cone  of  rays.  The  experiment  is  an  in- 
structive one,  and  can  be  easily  made  by  using  the  con- 
denser in  a  way  that  needs  no  description. 


Black-ground   Illuminators. 

This  form  of  illumination  as  accomplished  by  the 
sub  stage  condenser  is  not  entirely  satisfactory, 
although  the  effect  may  be  moderately  well  obtained  by 
that  accessory  with  low-power  objectives.  It  may  be 
better  done  with  appliances  specially  designed  for  the 
purpose,  all  of  which  deflect  the  illuminating  rays  so 
that  they  pass  beyond  the  objective  without  entering  it 
directly. 


l82  MICROSCOPICAL    PRAXIS. 

The  Paraboloid. 

This  consists -of  a  solid  parabola  of  glass  whose  sides 
are  so  curved  that  the  rays  of  light  which  impinge 
upon  them  are  internally  reflected,  so  that  they  escape 
from  the  front  at  such  an  angle  that  they  pass  beyond 
the  front  of  the  objective.  The  anterior  surface  is 
concave,  the  back  one  flat,  and  passing  through  the 
longitudina.1  axis  there  is,  in  some  forms,  a  movable 
.stop  formed  of  a  flat  disk  at  the  summit  of  a  stem. 
The  appliance  is  fitted  into  the  sub-stage  ring,  the  light 
reflected  from  the  plane  mirror,  and  the  instrument 
focussed  on  the  object.  It  gives  excellent  black- 
ground  illumination  with  magnifying  powers  up  to  the 
one-fourth  inch  objective.  When  the  central  stop  is 
present  it  should  be  pushed  up  when  the  paraboloid  is 
used  with  a  high-power  lens  and  depressed  for  a  low- 
power. 

It  may  be  used  as  an  immersion  instrument,  as  sug- 
gested by  an  anonymous  writer,  who  then  places  the 
microscope  in  a  vertical  position,  and  having  greased 
the  stem  of  the  stop  to  prevent  the  water  from  run- 
ning down  by  its  side,  the  hollow  is  filled  and  brought 
into  immersion  contact'  with  the  under  surface  of  the 
slide.  ''With  the  highest-power  objective  generally 
used  with  black-ground  illumination,  as  a  one-fourth  of 
from  75°  to  110°,  the  object  seems  no  brighter  than 
usual,  but  the  field  is  free  from  the  foggy,  diffuse  light 
otherwise  present,  and  the  object  appears  beautifully 
distinct  upon  a  jet-black  ground.  Even  a  one-fifth 
or  a  one-eighth  of  130°  gives  the  same  effect  of  a  deep 
black  back-ground  and  shows  the  object  with  good 
stereoscopic  effect  in  Wenham's  binocular.  With  objec- 
tives of  170°,  the  main  effect  is  that  of  a  dark  back- 
ground, though  not  so  perfect  as  with  the  lower 
angles." 


MICROSCOPICAL    PRAXIS.  183 

There  is,  however,  an  immersion  paraboloid  devised 
by  Mr.  Wenham  for  use  with  powers  higher  than 
the  one-fourth.  This  is  similar  to  the  foregoing,  but 
the  front  surface  is  flat  and  the  more  readily  to  receive 
the  drop  of  immersion-fluid,  and  black-ground  effects 
are  said  to  be  obtained  with  objectives  of  comparatively 
wide  angular-aperture. 


The  Spot  Lens. 

This  is  a  large  hemispherical  lens  upon  whose  plane 
surface  is  a  central  opaque  stop  to  intercept  all  the  rays 
except  those  passing  through  the  peripheral  parts.  It 
is  attached  in  various  ways  to  the  sub-stage,  so  that 
it  may  be  focussed  on  the  object,  which  is  illuminated 
by  the  reflection  from  the  cover-glass.  It  cannot  be 
satisfactorily  used  with  objectives  higher  in  power  than 
the  one-half  inch.  With  daylight  or  with  parallel  rays 
obtained  by  a  bull's-eye  condenser,  the  plane  mirror 
should  be  used  with  this  apparatus. 


184  MICROSCOPICAL    PRAXIS. 

The  Woodward  Prism. 

For  oblique  illumination  in  the  study  of  diatoms, 
test  objects  and  other  preparations  where  delicate  de- 
tails are  to  be  made  out,  the  Woodward  prism  is  a  useful 
and  simple  appliance  consisting,  as  first  suggested  by 
Dr.  J.  J.  Woodward,  of  a  right-angled  prism  attached  by 
immersion  contact  to  the  lower  surface  of  the  slide. 
The  original  device  was  for  use  with  certain  first-class 
objectives  in  connection  with  the  study  of  their  an- 
gular aperture,  but  the  opticians  have  so  modified  it 
that  it  is  is  now  a  useful  little  thing  under  lenses  of 
very  moderate  aperture. 

The  upper  face  of  the  prism  receives  a  drop  of 
glycerine  and  is  then  applied  to  the  slide  according  to 
the  way  in  which  it  has  been  mounted,  different 
opticians  adopting  different  methods.  The  simplest 
is  to  have  the  little  prism  without  metal  mounting  of 
any  kind,  when  it  is  to  be  attached  bodily  to  the  slide, 
the  drop  of  immersion-fluid  holding  it  in  place. 
If  it  should  slip  out  of  position,  as  it  probably 
will  when  the  stand  is  inclined,  a  strip  of 
paper  pasted  along  the  slide  will  hold  it  securely.  The 
prism  is  usually  about  half  an  inch  long  and  half  that 
length  in  width,  its  small  size  making  it  somewhat 
troublesome  to  manipulate,  unless  it  is  mounted  for 
attachment  to  the  stage  or  to  the  sub-stage. 

It  should  be  accurately  centred  to  the  objective,  and 
the  lower  edge  placed  as  nearly  vertical  as  possible  when 
seen  through  a  low-power.  Then  attach  the  higher 
power  to  be  used,  swing  the  mirror  to  one  side,  using 
the  bull's-eye  condenser  or  a  microscope-lamp  to  obtain 
parallel  rays,  and  illuminate  the  field.  The  best  effects 
obtainable  cannot  be  indicated  here;  the  reader  must 
seek  for  them  by  experimenting  with  the  mirror,  the 
prism  and  the  light. 


MICROSCOPICAL    PRAXIS.  185 

Ex-Gov.  J.  D.  Cox,  speaking  of  this  little  accessory  says: 
It  may  be  safely  asserted  that  for  whatever  purpose 
the  Wenham  reflex  illuminator  is  useful,  the  Woodward 
prism  is  superior;  and  we  have  no  hesitation  in  pre- 
dicting that  it  will  eventually  supersede  the  paraboloid 
for  black-ground  illumination  with  low-powers.  Indeed 
there  is  hardly  one  of  the  elaborate  and  expensive  sub- 
stage  illuminators  whose  work  cannot  be  better  done 
by  this  amusingly  simple  accessory,  which  can  be 
so  cheaply  made  as  to  form  part  of  every  outfit  for  a 
'Students's  Microscope,'  being  in  fact  less  costly  than 
the  very  cheapest  achromatic  condenser  furnished 
with  an  educational  instrument. 

The  late  Dr.  Allen  Y.  Moore  was  in  the  habit  of 
using  the  prism  for  the  illumination  of  opaque  objects 
under  high-power  objectives,  by  attaching  it  with  a 
drop  of  glycerine  between  its  broad  side  and  the  upper 
surface  of  the  slide.  To  accomplish  the  effect  sought  the 
object  must  be  mounted  dry  on  the  caver  glass,  a  dry 
objective  should  be  used  and  the  cement  ring  made  of 
Canada  balsam  or  some  other  transparent  material. 


Fig  12.     Illumination  of  opaque  objects  by  the  Woodward   prism. 

In  the  figure  (Fig.  12),  E  is  the  objective,  B  the  cover- 
glass,  C  the  cement  ring,  A  the  slide,  D  the  prism,  and 
F  and  G  two  rays  of  light  whose  course  is  shown  by 


l86  MICROSCOPICAL    PRAXIS. 

the  dotted  lines.  The  rays  passing  through  the  prism 
are  reflected  to  and  fro  by  the  surfaces  of  the  glass, 
brilliantly  illuminating  any  transparent  object  that 
may  be  properly  mounted  there.  Dr.  Moore  states 
that  he  has  used  this  method  with  a  magnifying  power 
of  four-thousand  diameters,  having  plenty  of  light  and 
good  definition. 


The  Hemispherical  Lens. 

This  is  simply  a  solid  hemisphere  of  glass  attached 
by  -means  of  a  drop  of  immersion-fluid  on  its  plane  sur- 
face to  the  lower  aspect  of  the  slide.  It  is  used  to 
concentrate  on  the  object  oblique  light  from  the  mirror 
when  the  latter  is  swung  to  one  side.  It  may  be  pre- 
vented from  slipping  when  the  microscope  is  inclined, 
in  the  way  suggested  for  the  same  purpose  with  the 
Woodward  prism.  A  better  method,  however,  is  to  use 
only  a  small  amount  of  water,  glycerine  or  whatever 
the  immersion  fluid  may  be. 

Its. optical  action  is  such  that  the  object  is  practi- 
cally at  the  centre  of  a  hemisphere,  where  the  light  is 
also  brought  to  a  focus,  the  lens  being  in  effect  ex- 
tended by  the  immersion-fluid  to  include  the  slide  and 
the  object.  It  is  used,  as  are  so  many  of  these  illumi- 
nating accessories,  in  the  resolving  of  diatoms  or  of 
other  finely  lined  tests,  and  to  assist  the  objective  in 
the  work.  For  other  purposes  it  is  of  little  or  no  im- 
portance. 


MICROSCOPICAL    PRAXIS.  187 

Supra-stage  Illuminators. 

There  are  many  pieces  of  apparatus  for  illumination 
from  above  the  object,  but,  as  with  those  for  sub-stage 
use,  the  majority  will  be  here  omitted.  Some  have 
been  forgotten  and  are  not  worth  recalling.  Others  are 
so  seldom  used  that  they  are  rapidly  on  their  way  to 
oblivion,  and  are  rarely  seen  anywhere  except  in  the 
opticians'  lists.  Among  these  is  the  Lieberkuhn. 


The  Lieberkuhn. 

This  is  a  cup-shaped  metal  reflector,  its  inside  sur- 
face polished  so  as  to  direct  the  light  upon  an  opaque 
object  after  it  has  been  attached  to  the  objective,  over 
which  it  is  slipped  by  means  of  a  collar.  It  was  de- 
vised in  1738  by  Johann  Lieberkuhn,  a  German  anato- 
mist and  microscopist  for  whom  it  is  named.  At  one 
time  it  was  extensively  employed,  but  it  has  so  many 
objectionable  features  that  it  has  now  fallen  into  disuse, 
although  its  effects  are  often  admirable. 

It  must  have  a  special  focus  and  therefore  a  special 
curvature  for  every  objective  for  which  it  is  intended, 
and  these  are  usually  low-powers. 

Each  objective  must  therefore  have  its  own  Lieber- 
kuhn. In  using  it,  the  light  must  be  intercepted  by  a 
central  opaque  disk,  or  a  dark-well,  so  that  no  rays 
shall  reach  the  object  directly,  but  pass  around  it  to 


l88  MICROSCOPICAL    PRAXIS. 

the  Lieberkuhn  thence  to  be  reflected  downward.  Con- 
sequently the  object  must  not  be  too  large.  Intense 
surface-illumination  may  be  obtained  equally  well, 
much  better  in^  some  respects,  by  other  means,  the 
Lieberkuhn  therefore  being  a  device  that  the  microsco- 
pist  can  get  along  without.  It  is  interesting  chiefly  on 
account  of  its  age  and  history. 


The  Parabolic  Speculum. 

This  too  has  gone  out  of  fashion,  but  it  might  well  be 
revived  for  use  with  low-powers  up  to  the  one-half  inch, 
and  over  large  opaque  objects.  It  is  a  silvered  and 
polished  surface  of  such  curvature  that  parallel  rays 
are  reflected  from  it  to  a  focus  on  the  object,  after  the 
apparatus  has  been  attached  to  the  objective,  and  the 
mirror  swung  above  the  stage.  A  large  object  may  be 
examined  with  all  the  shadow  effects  preserved  and  in- 
tensified, and  these  effects  are  in  some  cases  exceed- 
ingly fine. 

Mr.  E.  H.  Griffith  has  suggested  the  use  of  a  silver- 
plated  spoon  as  a  cheap  and  effective  substitute  for  the 
opticians'  finely  finished  accessory.  He  winds  a  clean 
copper-wire  of  one-twenty-fourth  inch  in  diameter  three 
times  around  the  base  of  the  objective,  bending  both 
ends  so  that  they  may  reach  somewhat  beyond  the  front 
of  the  lens.  He  than  cuts  a  section  of  about  half  an 


MICROSCOPICAL    PRAXIS.  189 

inch  from  the  bowl  of  a  new  tea-spoon,  and  solders  the 
ends  of  the  wire  to  the  convex  surface.  .The  wire  loop 
serves  to  hold  the  apparatus  in  position,  easily  sliding 
on  and  off  the  objective,  and  being  readily  bent  as  the 
adjustment  of  the  spoon-speculum  may  require.  This 
is  to  be  used  with  parallel  rays,  as  is  the  more  expen- 
sive but  scarcely  more  effective  parabolic  speculum 
supplied  by  the  opticians. 


The  Vertical  Illuminator. 

With  the  vertical  illuminator  the  objective  acts  not 
only  as  an  objective  but  as  an  achromatic  condenser, 
focussing  on  the  object  the  light  sent  through  it  from 
the  rear,  opaque  objects  being  seen*  therefore,  by  sur- 
face illumination,  and  transparent  substances  by  reflec- 
tion from  their  inferior  surface,  the  light  being  forced 
to  act  so  that  the  transparent  object  appears  self- 
luminous. 

The  apparatus,  which  was  invented  by  Prof.  Hamilton 
L.  Smith,  of  Geneva,  N.  Y.,  consists  of  a  hollow,  brass 
cylin'der  fitted  at  one  end  for  attachment  to  the  body- 
tube,  and  at  the  other  to  carry  the  objective.  At  one 
side  a  projecting  milled-head  bears  a  pin  which  carries, 
within  the  hollow  of  the  cylinder,  a  disk  of  thin  cover- 
glass  which  is  to  act  as  the  reflector,  to  throw  upon  the 
back  lens  of  the  objective. the  light  received  through  a 
lateral  opening  about  one-fourth  inch  in  diameter  and 


igo  MICROSCOPICAL    PRAXIS. 

opposite  the  glass  disk.  At  the  side  of  this  aperture, 
a  diaphragm  is  frequently  attached  so  that  by  it  the 
amount  and  the  direction  of  the  entering  light  may  be 
altered.  As  originally  devised  the  reflector  was  of 
metal,  but  this  has  been  replaced  by  the  equally  effec- 
tive thin-glass. 

With  the  vertical  illuminator,  opaque  objects  may  be 
studied  with  high-power  immersion-objectives.  Yet 
even  with  them  its  use  is  limited,  since  the  object  must 
be  mounted  dry  and  adherent  to  the  cover.  It  can  be 
utilized  for  the  examination  of  the  surface  of  blood- 
corpuscles,  diatoms,  insect  scales,  ruled  lines,  or  similar 
objects  capable  of  being  dried  without  injury,  or 
naturally  existing  in  a  dry  condition.  It  is  reported 
that  good  results  have  also  been  obtained  in  the  ex- 
amination of  mucus,  pus  and  liquids  containing  bac- 
teria, etc.;  also  in  the  minute  structure  of  muscle  and 
of  nerve-fibres. 

To  use  it,  the  objective  is  attached  at  the  lower  end 
and  focussed.  The  light  from  the  mirror  is  then  re- 
moved, and  the  illuminator  rotated  until  the  lateral 
aperture  faces  the  lamp,  whose  flame  should  be  on  a 
level  with  the  opening,  so  that  its  rays  may  enter  and 
fall  upon  the  glass  disk  within.  By  manipulating  this 
disk  by  means  of  the  external  milled-head,  the  light 
may  be  thrown  obliquely  or  centrally  on  the  back  lens 
of  the  objective,  through  which  it  passes  to  be  con- 
densed on  the  object  at  the  focus,  where  it  will  appear 
as  a  narrow  transverse  band.  The  objective  should  be 
carefully  adjusted  and  focussed,  and  the  band  of  light 
made  as  narrow  and  brilliant  as  possible.  The  best 
condition  of  the  light  can  be  obtained  only  by  experi- 
menting with  the  lamp,  the.  glass  disk  and  the  dia- 
phragm zt  the  lateral  opening. 


MICROSCOPICAL    PRAXIS.  19! 

Mr.  Adolph  Schultze,  writing  in  the  "Journal  of  the 
Quekett  Microscopical  Club,"  with  reference  to  the  use 
of  this  accessory,  says: — After  having  roughly  focussed 
the  lens  on  the  slide,  adjust  the  lamp  in  a  vertical  direc- 
tion so  that  a  line  perpendicular  to  the  optical  axis  of 
the  microscope,  drawn  through  the  centre  of  the  aper- 
ture of  the  vertical  illuminator,  passes  through  the  low- 
est point  of  the  flame,  .or  just  over  the  top  of  the  wick. 
Adjust  now  the  reflecting  surface  of  the  vertical  illumi- 
nator on  its  horizontal  axis,  so  that  a  distinct  image  of 
the  flame  appears  in  the  field  of  vision.  This  image 
will  of  course  appear  brightest,  and  the  definition  best, 
when  the  narrowest  side  of  the  flame  is  turned  toward 
the  instrument,  which  can  be  easily  ascertained  by  turn- 
ing the  oil-vessel  of  the  lamp  a  little  round  its  axis, 
whilst  looking  into  the  microscope.  The  field  is  now 
quite  dark,  and  nothing  is  seen  but  a  streak  of  light 
about  a  quarter  of  an  inch  in  breadth,  which  passes 
through  the  middle  of  it  antero-posteriorly.  If  all  this 
has  been  carefully  attended  to,  and  if  a  diatom,  adher- 
ing closely  to  the  cover,  is  moved  into  this  image  of  the 
flame,  its  markings  will  appear  most  beautifully  and 
distinctly  resolved,  provided  they  are  lying  across  the 
path  of  the  light.  Various  little  advantages  may  be 
gained  by  a  more  careful  regulation  of  the  relative 
heights  of  the  light  to  that  of  the  aperture  of  the  verti- 
cal illuminator,  or  by  shutting  off  one-half  of  the  aper- 
ture of  the  latter,  or  by  allowing  the  light  to  fall  on  the 
lower  half  of  the  aperture,  etc.,  all  of  which  have  for 
their  object  to  let  the  light  fall  on  only  one-half  of  the 
reflecting  surface,  leaving  the  other  for  tt;e  passage  of 
the  rays  from  the  object  to  the  eye-piece.  The  image 
of  the  flame  does  not  always  intersect  the  whole  of  the 
field,  and  in  this  case  it  falls  more  in  the  fore  part. 


192 


MICROSCOPICAL    PRAXIS. 


By  means  of  this  apparatus  the  lines  on  the  nineteenth 
band  of  Nobert's  test-plate  have  been  separated,  and 
Amphipleura  pellucida  has  been  resolved  into  dots  by 
the  proper  objectives. 


Fig.  ig.     The  vertical  illuminator. 

It  is  exceedingly  difficult  to  use,  but  its  effects  on 
the  proper  objects  are  remarkable.  It  is  shown  in  Fig. 
13,  A  being  the  instrument,  B  the  milled-head  pin  to 
hold  the  thin  cover-glass,  and  C  a  sectional  view  as  at- 
tached to  the  microscope  and  objective,  D  being  the 
reflector  and  the  parallel  lines  the  entering  and  re- 
flected rays. 


MICROSCOPICAL    PRAXIS.  193 

The  Amplifier. 

The  amplifier  is  a  double  concave,  or  an  achromatic 
concavo-convex  lens  placed  in  the  tube  of  the  micro- 
scope between  the  eye-piece  and  the  objective,  to  in- 
crease the  size  of  the  image  formed  by  the  latter,  and 
so  apparently  to  increase  the  magnifying  power.  The 
accessory  has  long  been  used  in  the  telescope,  and  for 
many  years  to  a  certain  extent  in  the  microscope.  Yet, 
although  known,  it  did  not  come  into  even  limited  em- 
ployment until  the  late  R.  B.  Tolles  produced  his  achro- 
matic amplifier,  and  even  since  that  time  it  has  not  been 
very  extensively  used  although  this  achromatic  form  is 
immensely  superior  to  the  common  double-concave  lens 
sometimes  employed,  and  which  is  accompanied  with 
considerable  loss  of  light  and  impairment  of  definition. 

Mr.  Tolles's  lens  is  so  mounted  that  it  may  be 
screwed  into  the  draw-tube.  It  is  said  that  by  its 
means  resolution  has  been  had  of  the  nineteenth  band 
of  the  Nobert  test-plate,  which  could  not  be  had  with- 
out it  by  the  same  objective,  and  that  it  does  not  neces- 
sitate any  readjustment  of  the  objective,  and  only  the 
slightest  alteration  of  the  focus. 

Dr.  G.  Devron  of  New  Orleans,  in  an  enthusiastic  de- 
scription of  this  special  form,  says: — Every  microsco- 
pist  should  possess  a  Tolles's  amplifier,  or  a  similar  in- 
strument; its  cost  is  but  little  more  than  that  of  an  or- 
dinary eye-piece,  and  as  it  may  be  used  with  every  eye- 
piece, its  possession  is  equal  to  having  twice  as  many 
such  glasses;  and  the  possessor  of  a  good,  modern  ob- 
jective of  moderate  power  can  accomplish  with  it  al- 
most anything  that  would  require  an  objective  of  the 
same  grade  and  of  double  magnifying  power;  thus  a 
one-twelfth  with  an  amplifier  will  do  the  same  work 
that  a  one-twenty-fourth  would  do  without  the  ampli- 

14 


194  MICROSCOPICAL    PRAXIS. 

fier.  Object  glasses  of  high-power  are  very  expensive 
when  well  made,  and  require  great  manipulative  skill 
for  their  use,  while  a  medium  power  of  the  same  quality 
is  not  half  as  expensive,  is  easily  worked  and  is  more 
•frequently  needed.  Other  microscopists  do  not  agree 
with  Dr.  Devron,  condemning  the  apparatus  for  de- 
stroying some  of  the  objective's  definition. 

Personally  I  have  never  used  Mr.  Tolles's  or  any 
other  amplifier.  With  it  the  late  Dr.  J.  J.  Woodward 
of  Washington,  and  Mr.  G.  W.  Rafter  of  Rochester,  N. 
Y.,  have  obtained  excellent  results  in  photo-microg- 
raphy. 


The  Erector. 

The  inversion  of  the  image  renders  dissection  or  in- 
deed any  manipulations  with  the  compound  microscope, 
difficult  to  most  persons,  as  they  are  unable  to  cultivate 
readily  the  habit  of  moving  the  hand  towards  the  left 
when  it  is  desired  to  have  the  image  move  toward  the 
right,  or  vice  versa.  The  opticians  therefore  offer  a 
prism  of  peculiar  form,  so  mounted  that  it  may  be 
placed  between  the  ocular  and  the  objective,  and  by  its 
action  erect  the  image,  thus  constituting  what  is  called 


MICROSCOPICAL    PRAXIS.  195 

the  erector.  Mr.  Joseph  Zentmayer  makes  one  which 
is  said  to  interfere  very  little  with  the  definition  of  the 
objective,  and  Messrs.  Bausch  and  Lomb  add  a  body- 
tube  to  their  dissecting  and  mounting  microscope  in 
which  is  an  erecting  prism. 

An  anonymous  writer  describes  a  home-made  erector 
formed  from  one  of  the  right-angled  prisms  which  are 
used  for  ornament  around  certain  chandeliers.  If  this 
be  held  horizontally  over  the  eye-piece,  with  the  widest 
face  directed  from  the  observer,  and  the  image  viewed 
through  it,  the  object  will  seem  to  be  in  its  normal  po- 
sition; and  if  the  prism  be  held  in  exactly  the  right  po- 
sition, the  definition  will  be  slightly  if  at  all  impaired. 

I  have  experimented  with  this  simple  contrivance 
with  objectives  up  to  the  half-inch,  and  nothing  above 
this  is  ever  used  in  ordinary  dissections,  and  I  find  that 
it  does  all  that  could  be  expected  of  it,  and  with  but 
little  deterioration  in  the  definition.  It  would  not  be  a 
difficult  task  for  anyone  possessing  even  a  little  me- 
chanical ability,  to  cut  off  a  portion  of  such  a  prism 
and  to  mount  it  above  the  eye-piece,  after  the  cap  had 
been  removed,  if  necessary. 

The  image  may  also  be  erected  by  the  use  of  an  ob- 
jective placed  over  the  ocular,  the  screw-end  down- 
ward, or  in  the  draw-tube  above  the  objective.  I  have 
obtained  excellent  results  without  much  impairment  of 
the  definition,  by  using  Messrs.  J.  W.  Queen  &  Cov's 
two-inch  objective  in  the  draw-tube  with  the  front  lens 
directed  upward,  with  Spencer's  one-inch  of  33°  on  the 
body-tube.  The  working  distance  of  the  one-inch  is 
thus  greatly  increased,  the  magnification  reduced,  and 
the  field  contracted  in  size,  but  various  manipulations 
about  the  stage  may  be  accomplished  with  the  great- 
est comfort  under  the  combination. 


196  MICROSCOPICAL    PRAXIS. 

The  Polariscope. 

This  apparatus,  so  important  in  the  study  of  crystals 
and  of  the  structure  of  rocks,  divulging  secrets  that 
could  be  learned  by  no  other  means,  consists  of  two 
prisms  of  Iceland  spar,  one  placed  below  the  object 
and  called  the  polariser,  the  other,  the  analyser,  above 
it,  usually  with  a  plate  of  selenite  on  the  stage  beneath 
the  object.  The  use  of  ordinary  light  may  show  noth- 
ing of  the  organization  of  certain  bodies,  which  is  at 
once  revealed  when  the  polariscope  begins  to  question 
it.  In  the  study  of  crystallography  and  of  petrology  it 
is  indispensible.  And  for  the  revelation  of  that  per- 
fection and  richness  of  color  so  appreciated  by  the  nor- 
mal eye,  it  is  the  only  means  that  we  have  for  its  exhi- 
bition in  the  microscope. 

Polarisation  of  light  was  discovered  by  accident  in 
1808.  Etienne  Louis  Malus,  a  French  optician,  while 
experimenting  with  light,  happened  to  view  through  a 
•doubly  refracting  prism  of  Iceland  spar,  the  light  of 
the  setting  sun  as  it  was  reflected  from  a  glass  door 
which  was  standing  open.  The  phenomenon  then  seen 
for  the  _first  time  he  named  polarisation,  because,  on 
further  study,  he  supposed  that  certain  of  its  properties 
bore  "some  analogy  to  the  opposite  properties  of  the 
different  poles  of  a  magnet,"  so  that  this  peculiar  kind 
of  light  was  said  to  be  polarised.  The  supposition, 
however,  was  an  entirely  gratuitous  one. 

The  effect  is  produced  by  "light  reflected  from  or 
transmitted  through  glass  at  an  angle  of  incidence  of 
54°  35'»  or  light  propagated  by  only  one  plane  of  vi- 
brations." It  is  supposed  that  a  ray  of  common  light 
consists  of  vibrations  in  all  possible  planes,  but  for 
practical  purposes  they  are  considered  to  be  in  only  two 
directions  at  right-angles. to  each  other,  polarised  light 


MICROSCOPICAL    PRAXIS.  197 

being  that  kind  in  which  the  vibrations  are  reflected  or 
transmitted  in  one  plane  only,  all  the  other  kind  of  vi- 
brations having  by  some  means  been  intercepted. 
Prof.  A.  P.  Gage  in  his  "Elements  of  Physics,"  illus- 
trates this  by  Fig.  14,  saying  that  the  action  of  the 


Figure  14.    "Explanation  of  polarised  light. 

tourmaline  (the  polarising  substance  used  in  the  experi- 
ment) may  be  compared  to  that  of  a  grating  A'  in  the 
figure,  formed  of  parallel  vertical  rods,  which  will  al- 
low all  vertical  planes,  as  C  C'  to  pass,  but  stops  the 
planes  D  D'  that  are  at  right-angles  to  these  rods.  Any 
plane  that  has  succeeded  in  passing  one  grating  will 
readily  pass  a  second  similarly  placed.  But  if  the 
second  grating  j9,  is  turned  so  that  its  rods  are  at 
right-angles  to  the  first,  the  plane  that  has  succeeded 
in  getting  through  the  first  grating  will  be  stopped  by 
the  second. 

Iceland  spar  is  the  substance  used  for  the  microscop- 
ical polariscope,  being  a  mineral  not  rarely  found  m 
many  countries.  It  is  a  colorless,  entirely  transparent 
carbonate  of  lime,  and  capable  of  being  separated,  by 
the  proper  treatment,  into  a  six-sided  solid  of  a  peculiar 
inclined  form,  called  a  rhomb  of  Iceland  spar.  From 
this  are  made  the  Nicol  prisms,  being  so  named  for  a 
Mr.  Nicol,  an  optician  of  Edinburgh,  who  first  pre- 
pared them  by  dividing  these  rhombs  into  two  equal 
parts  by  a  plane  passing  at  a  certain  angle  diagonally 
through  them. 


198  MICROSCOPICAL    PRA*XIS. 

In  the  undivided  rhomb  a  ray  of  light  falling  perpen- 
dicularly on  the  surface  is  divided  into  two  separate 
parts,  one  taking  the  direction  of  the  original  ray,  and 
so  named  the  "ordinary  ray,"  the  other  being  refracted 
to  form  what  is  called  the  "extraordinary  ray,"  so  that 
any  small  object  looked  at  through  a  slice  of  Iceland 
spar,  will  appear  to  be  doubled.  Mr.  Nicol,  by  his  treat- 
ment of  the  rhomb,  succeeded  in  producing  single-image 
prisms  for  use  with  the  polariscope,  in  which  there  are 
two,  one  forming  the  polariser,  the  other  the  analyser, 
the  extraordinary  ray  alone  being  allowed  to  pass, 
the  ordinary  suffering  total  reflection. 

The  polarising  prism  is  so  mounted  that  it  may  be  at- 
tached to  the  sub-stage  or  inserted  into  the  stage-open- 
ing. It  must  be  below  the  object,  and  either  it  or 
the  analyser  must  be  capable  of  rotation  at  the  will  of 
the  microscopist,  but  which  shall  be  movable  is  of  no 
importance.  For  convenience,  the  polariser  may  be 
used  with  the  sub-stage  condenser  and  below  it. 

The  analyser  must  be  above  the  object.  Whether  it 
is  placed  in  the  body-tube  or  above  the  eye-piece  has 
some  effect  upon  the  intensity  of  the  illumination  and 
the  size  of  the  field,  but  none  upon  the  polarising  action. 
If  the  analyser  is  in  the  body-tube  the  light  is  dimin- 
ished; if  above  the  eye-piece  the  field  is  greatly  reduced 
in  size,  while  the  amount  of  light  suffers  little  or  no 
diminution. 

When  the  prisms  are  applied  to  the  microscope,  they 
should  be  in  such  a  position,  that  is,  with  their  polaris- 
ing planes  parallel  with  each  other,-  tha't  thefield  shall 
be  brightly  illuminated.  If  it  should  be  darTorVonly 
faintly  lighted,-the  prisms  are  at  or.  near -right-angles  to 
each  other,  and  are  'then  ^saitf  to  be  grossed  and  th^ 
light  may^be  turned 'on  by  rotating  one  or  the  other 


MICROSCOPICAL    PRAXIS.  199 

Inmost  work  with  the  polariscope  it  is  usually  best  to 
use  a  low-power  objective  only,  which  is  attached  to  the 
body  in  the  usual  way,  the  field  illuminated  and  the  ob- 
ject focussed.  The  polariser  or  the  analyser  is  rotated 
and  the  effect  studied.  There  is,  however,  nothing  in 
the  polariscope  to  prevent  its  use  with  high  power-ob- 
jectives over  suitable  objects. 

When  the  prisms  are  crossed  at  right  angles  to  each 
other,  the  field  will  be  absolutely  black  if  the  apparatus 
is  perfect,  and  with  the  field  thus  darkened,  any  trans- 
parent object  capable  of  being  polarised,  as  all  sub- 
stances are  not,  will,  when  placed  under  the  objective, 
be  lighted  and  may  be  exquisitely  colored.  Yet  some 
objects,  although  they  may  show  the  effect  of  polarised 
light  when  the  prisms  are  rotated,  may  not  be  colored. 
In  such  cases  a  plate  of  selenite  is  placed  beneath  it, 
when  it  will  exhibit  at  every  quarter  turn  of  the  prism, 
complementary  colors  which  will  vary  according  to  the 
thickness  of  the  selenite. 

The  instrument  deserves  to  come  into  more  general 
use.  It  is  employed  with  ease,  demanding  no  compli- 
cated or  delicate  manipulations,  and  its  effects  are  al- 
ways pleasing  and  such  as  can  be  obtained  only  by  it. 
That  it  is  not  oftener  employed  by  microscopists  is 
probably  due  to  its  cost,  and  probably  to  a  feeling  that 
its  practical  usefulness  is  not  sufficient  to  warrant  the 
outlay;  but  this  is  a  mistake.  It  merits  careful  study. 


200  MICROSCOPICAL    PRAXIS. 

Drawing  the  Object. 

The  microscopist  that  cannot  draw  is  unfortunate. 
There  are  many  occasions  when  to  be  able  to  sketch  the 
appearances  seen  at  the  moment,  is  a  great  convenience, 
whereas  to  be  compelled  to  attach  the  camera  lucida  to 
the  eye-piece,  and  to  prepare  the  paper  and  the  light, 
not  only  consume  time,  but  may  result  in  the  loss  of 
the  object  if  it  be  a  living  one.  All  forms  of  camera 
lucidas  are  intended  for  use  in  drawing  the  outlines 
only  of  the  object.  The  shading  and  the  fine  artistic 
finish  must  be  added  afterwards.  All  the  devices  are 
attached  to  the  ocular  above  the  eye-lens  after  the  cap 
has  been  removed,  and  considerable  practice  is  required 
with  all  the  different  kinds  before  they  can  be  used 
with  much  success. 

The  light  must  be  correctly  modified  or  the  pencil 
point  will  be  invisible.     This  is  one  of  the  necessities, 
and  the  invisibility  of  the  pencil  point  one  of  the  diffi- 
culties in   the  use  of  all   these  drawing  contrivances. 
The  light  on  the  paper  should  not,  as  a  rule,  be  strongeA 
than  that  on  the  object;  it  should  usually  be  weaker./ 
If  it  must  be  increased,  as  it  must  with  certain  forms, 
the  lamp  can  be  arranged  so  that  it  may  be  nearer  to 
the    paper,  and  a  bull's-eye   condensing  lens  be  inter- 
posed between  them,  while  the  illumination  of  the  ob- 
ject may  be  modified  by  diaphragms  or  by  increasing 
the  distance  between  it  and  the  mirror,  if  the  concave 
face  is  used.     The  pencil  should  be  rather  soft,  with  a\ 
long,  sharp   point,  whose  visibility   some   microscopists   \ 
are  in  the  habit  of  attempting  to  increase  by  the  addi-     \ 
tion  of  a  little  tin-foil  wrapped  about  it,  contending  that 
the  bright  foil   is  more   easily  seen   than  the  graphite   J 
pencil   point.      Others  blacken   the  end  of   the  pencil    \ 
with  india  ink,  making  for  this  the  same  claim  as  for  ^ 


MICROSCOPICAL    PRAXIS.  2OI 

the  tin-foil.  The  proper  illumination  seems  to  be  more 
important  than  any  of  these  aids,  and  its  careful  regu- 
lation better  than  any  addition  to  the^fencil  point. 

Mr.  T.  Suffolk  describes,  in  "Science  Gossip,"  his 
method  of  drawing  without  a  camera  lucida  of  any  form. 
He  places  on  the  diaphragm  of  the  eye-piece  a  convex 
lens  of  shallow  curvature,  upon  the  surface  of  which  is 
a  grating  ruled  in  squares,  the  lines  being  about  one- 
twentieth  of  an  inch  apart.  The  drawing,  which  is 
made  on  paper  also  ruled  in  squares,  may  then  be  en- 
larged or  reduced  after  the  well-known  method  em- 
ployed by  draughtsmen.  "The  process  also  possesses 
the  additional  advantage  of  requiring  no  change  in  the 
position  of  the  microscope,  as  is  the  case  with  the  cam- 
era lucida,  and  can  be  used  for  a  long  time  without  any 
of  the  strain  on  the  eye  inseparable  from  the  use  of  in- 
struments where  the  image  and  pencil  point  are  viewed 
through  the  divided  pupil  of  the  eye." 


The  Thin-glass  Reflector. 

The  simplest  form  of  all  the  appliances  intended  for 
the  microscopist  that  cannot  draw  free-hand,  is  a  thin- 
glass  cover  attached  in  any  convenient  way  to  the 
ocular,  so  that  the  image  shall  be  received  upon  its 


202  .  MICROSCOPICAL    PRAXIS. 

upper  surface  at  the  proper  angle,  which  may  be  de- 
termined by  experiment,  but  which  is  usually  45  de- 
grees if  the  body-tube  becomes  horizontal  when  in- 
clined, as  it  will  not  become  on  all  stands.  The  micro- 
scope is  depressed  into  this  horizontal  position,  the 
paper  arranged  on  the  table  beneath  the  eye-piece,  and 
while  the  pencil  is  seen  through  the  thin  cover-glass, 
the  object  appears  to  be  projected  on  the  paper,  where 
the  pencil  draws  the  outlines. 

Mr.  T.  B.  Jennings  has  described  the  following  sim- 
ple method  of  mounting  the  reflector.  In  a  flat  cork  he 
cuts  a  central  opening  large  enough  to  pass  over  the 
mounting  of  the  eye-lens,  and  just  below  this  aperature 
he  makes  an  incision  to  receive  the  cover-glass  so  that 
it  may  stand  at  an  angle  of  45°. 


Beale's  Neutral  Tint  Reflector. 

A  form  similar  to  the  thin-glass  disk  but  differing  in 

being  tinted  a  neutral  color,  is  Beale's  reflector.      It  is 

•?%-•*«     employed  quite  extensively,  more  so  than  the  colorless 

''^Vcover-glass,  but   in  the  same  way.      It  is"  made  by  op- 

>"-•-..- -ticians    generally,    being   intended    to    slide   over    the 

'^-•^ Amounting  of  the  eye-lens,  after  the   cap    has   been  re- 

-  moved.  "• 


MICROSCOPICAL    PRAXIS.  203 

A  Simple  Mirror-reflector. 

An  anonymous  writer  recommends  a  reflector  to  be 
made  as  follows.  Take  a  small  portion  of  the  silvering, 
about  one-sixteenth  of  an  inch  in  diameter,  from  the 
back  of  a  mirror,  where  there  should  be  a  thick  coating 
of  paint  on  the  amalgam  to  support  it,  or  it  will  not 
break  off.  This  small  amalgam  reflector  is  to  be 
mounted  with  cement  on  a  piece  of  watch-spring  at  the 
proper  angle.  The  spring  is  bent  round  and  fixed  to  a 
brass  tube  fitting  over  the  eye-piece,  so  that  the  re- 
flector may  stand  about  one-fourth  of  an  inch  from  the 
eye-lens  and  central  with  it.  On  looking  into  it,  the 
object  on  the  stage  is  seen  and  appears  to  be  projected 
on  the  paper  below. 

The  objection  to  all  these  simple  forms,  especially  to 
the  neutral  tint  reflector  and  its  imitators,  is  that  they 
not  only  invert  the  image,  the  top  appearing  at  the 
lowest  part,  but  seem  also  to  turn  it  over,  the  left-hand 
side  being  transferred  to  the  right-hand.  To  finish  a 
drawing  thus  inverted  and  reversed  is  difficult.  The 
microscopist  will  be  sure  to  lose  his  way  when,  after 
glancing  through  the  microscope  for  the  details,  he 
attempts  to  add  them  to  his  sketch.  With  the  neutral 
tint  the  field  is  also  very  small.  To  see  the  whole  of 
the  field  at  one  time  is  impossible. 


2O4  MICROSCOPICAL    PRAXIS. 

Wollaston's  Camera  Lucida. 

One  of  the  earliest  forms  of  drawing-prisms,  and  one 
of  the  best,  is  the  four-sided  accessory  known  as 
Wollaston's  camera  lucida.  It  is  so  mounted  that  it 
slides  over  the  eye-lens,  after  the  cap  has  been  re- 
moved, the  image  being  apparently  projected  on  the 
paper  upon  the  table,  and  seen  in  its  proper  position, 
since  it  has  been  twice  reflected  by  the  prism,  the 
pencil  and  the  paper  being  seen  direct,  that  is,  without 
reflection.  The  acute  edge  of  the  prism  bisects  the 
pupil  of  the  eye,  one-half  receiving  the  rays  from  the 
microscope,  the  other  half  those  from  the  paper  and 
the  pencil.  For  this  reason  it  is  at  first  somewhat 
difficult  to  use,  but  a  little  practice  soon  conquers 
it.  Like  all  similar  kinds  of  apparatus  it  is  used  with 
the  left-eye  closed. 

The  microscope  must  be  in  a  horizontal  position,  and 
the  light  on  the  drawing-surface  should  be  much 
stronger  than  that  on  the  object.  When  the  illumina- 
tion has  been  properly  adjusted  the  prism  can  be  used 
with  great  facility,  and  most  satisfactory  drawings 
made  with  it.  If  the  field  projected  on  the  paper  below 
the  prism  is  not  evenly  illuminated,  or  if  it  is  colored 
at  the  margins,  the  camera  lucida  should  be  altered  by 
rotating  it  slightly,  or  by  changing  its  position  in  re- 
lation to  the  eye-piece.  When  the  object  is  focussed, 
and  the  field  as  seen'  on  the  paper  is  properly  illum- 
inated, the  eye  is  kept  steadily  over  the  acute  edge  of 
the  prism  while  the  pencil  sketches  the  outlines  of  the 
image.  The  secret  of  success  is  to  keep  the  eye  mo- 
tionless. If  it  turns  ever  so  slightly  the  pencil  point 
will  mysteriously  disappear,  in  which  event  it  should  be 
held  immovable  until  the  eye  again  finds  it. 

If  the  microscopist  is  accustomed  to  the  use  of  spec- 


MICROSCOPICAL    PRAXIS.  205 

tacles  he  will  find  them  an  advantage  when  drawing 
with  any  kind  of  camera.  One  of  the  troubles  experi- 
enced by  some  that  use  spectacles  is  that  the  pencil 
cannot  be  seen  at  the  same  time  with  the  object  when 
the  spectacles  are  removed.  Those  whose  eyes  are 
perfect  occasionally  have  the  same  trouble.  This  may 
be  imperfectly  remedied  by  using  a  very  low-power  con- 
vex lens  placed  immediately  below  the  prism,  between 
it  and  the  paper.  By  its  use  the  rays  from  the  drawing 
surface  are  given  the  same  degree  of  divergence  as  those 
from  the  camera.  Some  opticians  attach  this  lens 
movably  to  the  apparatus,  so  that  it  may  be  utilized  or 
not  as  the  microscopist  may  desire. 

The  size  of  the  drawing  will  depend  upon  the  dis- 
tance between  the  paper  and  the  prism,  increasing  with 
that  distance,  so  that  almost  any  enlargement  may  be 
obtained.  Immense  diagrams  may  be  made,  according 
to  Dr.  L.  S.  Beale's  method,  by  placing  the  paper  on  the 
floor,  or  what  may  be  more  satisfactory  for  ordinary  pur- 
poses, by  raising  the  microscope  on  a  box  or  other  sup- 
port above  the  table  on  which  the  drawing  is  to  be 
made. 


The  Abbe  Camera  Lucida. 

Perhaps  the  most  prominent  camera  now  offered  by 
the  opticians  is  that  of  Professor  Abbe.  It  consists  of 
two  prisms  cemented  together  so  as  to  form  a  cube, 


206  MICROSCOPICAL    PRAXIS. 

one  of  the  contacting  hypothenuse  surfaces  being 
silvered,  or  gilded,  with  the  exception  of  a  small  spot  in 
the  centre,  and  slightly  removed  from  the  prism  so  that 
there  is  a  thin  film  of  air  between  them.  A  plane  mir- 
ror at  the  end  of  a  lateral  arm  reflects  the  rays  from  the 
pencil  and  the  paper  to  the  silvered  prism,  whence  they 
are  reflected  to  the  eye. 

This  camera  may  be  used  with  the  microscope  either 
vertical  or  inclined,  the  best  effects  being  obtained 
when  the  body-tube  is  somewhat  inclined,  as  the  distor- 
tion of  the  image  is  then  less  than  when  the  micro- 
scope is  vertical.  When  the  prism  is  at  one  side  of  the 
instrument  and  the  paper  at  one  side  of  the  microscope- 
foot,  the  distortion  is  so  great  that  a  special  drawing- 
desk  must  be  used  and  inclined  at  an  oblique  elevation. 

It  is  to  Prof.  S.  H.  Gage,  of  Cornell  University,  that 
we  owe  a  more  complete  knowledge  of  the  distortion 
produced  by  this  camera  and  of  the  best  means  of  over- 
coming it.  When  using  this  form  the  aperture  in  the 
silvered  surface  of  the  prism  must  be  exactly  in  the  op- 
tic axis  of  the  microscope  and  at  just  the  right  distance 
above  the  eye-piece.  If  the  field  as  seen  through  the 
camera  is  not  evenly  illuminated,  or  if  the  entire  field 
can  not  be  seen  at  once,  the  prism  has  not  been 
properly  placed,  and  must  be  re-arranged  by  altering 
the  little  screws  in  the  collar  of  the  instrument.  The 
mirror  of  the  Abbe  cameras  which  I  have  seen  is  per- 
manently fastened  at  an  angle  of  about  45°,  so  that  the 
microscopist  is  relieved  from  all  manipulations  of  this 
part,  except  to  slide  it  to  and  fro  on  the  mirror-bar  so 
as  to  select  the  portion  of  the  drawing  paper  or  of  the 
table  that  is  to  be  reflected.  As  Prof  Gage  says  in 
reference  to  the  use  of  all  forms  of  camera  lucida: 
"In  order  that  the  picture  drawn  .  .  .  may  not  be  dis- 


MICROSCOPICAL    PRAXIS. 


207 


torted,  it  is  necessary  that  the  axial  ray  from  the  image 
on  the  drawing-surface  shall  be  at  right  angles  to  the 
drawing-surface."  And  that  the  end  of  the  drawing 
board  shall  be  in  a  plane  parallel  with  the  stage  of  the 
microscope;  the  mirror  must  also  have  its  edges  parallel 
with  the  edges  of  the  drawing-board. 

If  the  mirror  of  the  camera  be  movable  on  its  axis, 
when  the  microscope  is  inclined  the  mirror  must  be 
manipulated  so  as  to  prevent  the  distortion  otherwise 
inevitable.  The  general  rule  in  these  cases,  again  quot- 
ing from  Prof.  Gage,  is  to  raise  the  drawing-board 
twice  as  many  degrees  toward  the  microscope  as  the 
mirror  is  depressed  below  45°.  With  the  mirror  at  45° 
the  drawing-surface  should  be  horizontal;  with  the  mir- 
ror at  40°  the  drawing-surface  should  be  elevated  10°, 
and  with  the  mirror  at  35°  it  should  be  elevated  20° 
toward  the  microscope.  Even  in  these  conditions  there 
will  be  distortion  from  front  to  back  unless  the  drawing- 
board  is  again  elevated  in  the  proper  position.  To 
accomplish  all  these  points  Mrs.  Gage  has  devised 
a  drawing-board  shown  in  Fig.  15.  It  is  to  be  used  with 


Figure  15.     Mrs.  S.  H.  Gage's  drawing-desk  for  the  Abbe  camera  lucida. 

the  mirror  at  35°  and  the  microscope  inclined  at  30°. 
For  other  positions  of  the  camera-mirror  and  of 
the  microscope,  a  drawing-board  may  be  devised  with 
the  proper  surfaces  by  Prof.  Gage's  rule  already  given. 


208  MICROSCOPICAL    PRAXIS. 

Messrs.  Bausch  &  Lomb  make  a  camera  lucida  to  be 
used  on  the  microscope  when  inclined,  with  which  they 
claim  that  the  pencil-point  is  well  seen,  and  the  distor- 
tion less  than  with  even  the  Wollaston  prism.  I  have 
not  seen  it. 


The  Grunow  Camera  Lucida. 

In  this  device,  which  is  made  by  Mr.  J.  Grunow,  of 
New  York,  the  plane  mirror  of  the  Abbe  camera  is  re- 
placed by  a  rectangular  prism  to  reflect  the  rays  from 
the  drawing-surface.  It  consists  of  three  rectangular 
prisms,  two  forming  a  cube,  one  surface  being  silvered 
or  coated  with  gold,  and  having  a  perforation  in  this  de- 
posit essentially  as  in  the  Abbe  form,  the  third  prism 
being  intended  to  reflect  the  rays  from  the  paper.  The 
apparatus  may  be  used  with  the  body-tube  inclined  or 
vertical,  giving,  it  is  said,  a  very  distinct  image  of  the 
pencil  and  the  object.  "A  portion  of  the  surface  of  the 
work-table  of  the  size  of  about  twelve  or  fifteen  inches 
is  projected  into  the  field  of  view,  so  as  to  be  distinctly 
and  clearly  seen  with  the  object  on  the  stage."  The 
camera  must,  however,  be  especially  fitted  to  the  eye- 
piece with  which  it  is  to  be  used.  I  have  not  seen  it. 


MICROSCOPICAL    PRAXIS.  209 

Live-boxes,    Growing-cells,  and    other    Acces- 
sories. 

On  the  upper  left-hand  corner  of  many  microscope 
stages  will  be  noticed  a  small  hole  intended  to  receive 
the  stem  of  the  stage  forceps,  a  piece  of  apparatus 
sometimes  furnished  with  stand.  It  was  more  frequently 
added  by  the  older  opticians  than  it  is  by  those  of  the 
present  day,  and  it  was  oftener  used  by  the  older 
microscopists  than  it  is  by  those  of  the  present  time. 

It  consists  of  a  delicate  forceps  opened  by  a  slight 
pressure  on  a  projecting  pin,  closing  by  its  own  elastic- 
ity, and  with  a  long  handle  having  universal  motions 
by  means  of  a  ball-and-socket  joint,  or  by  some  other 
way.  It  is  used  to  hold  small  insects  or  opaque  objects 
under  the  objective,  generally  a  low-power,  so  that  they 
may  be  examined  in  all  their  parts  and  in  all  positions. 


The  Mechanical  Finger. 

The  microscopist  that  does  much  mounting  soon 
feels  a  desire  to  produce  some  preparation  to  show  his 
skill  in  the  manipulation  of  small  objects,  as  well  as  the 
adaptability  of  small  objects  to  the  formation  of  beauti- 
ful slides.  Butterfly  scales  are  arranged  into  bouquets 
of  the  most  delicate  and  gorgeous  hues;  diatoms  are 
laid  down  in  intricate  geometrical  patterns  and  figures 
that  are  a  joy  to  the  eye,  or  they  are  selected  from  a 
mass  of  dirt,  and  lifted  into  the  light  and  the  purity  of 
another  slide,  where  the"  expert  microscopist  studies 
them  with  increased  pleasure  when  he  recalls  the  source 


210  MICROSCOPICAL    PRAXIS. 

whence  he  took  them.  All  such  delicate  work  is  done 
by  the  use  of  a  mechanical  finger,  which  is  essentially  a 
fine  bristle,  or  a  filament  of  spun  glass,  attached  in 
some  way  below  the  objective.  The  filament  picks  up 
the  little  object  while  the  microscopist's  eye  is  at  the 
ocular,  holds  it  suspended  in  air  until  he  moves  a  slide 
beneath  it,  when  it  gently  lowers  the  selected  speci- 
men into  the  spot  prepared  for  it,  and  returns  under  the 
microscopist's  intelligent  guidance  for  another  similar 
load.  In  this  way,  after  repeated  visits  to  the  source 
of  supply,  the  mechanical  finger  produces  exquisite 
arrangements  of  diatoms,  of  insect  scales  or  of  other 
minute  objects. 

To  use  even  the  simplest  mechanical  finger  demands 
much  preliminary  practice,  yet  those  that  have  mas- 
tered the  apparatus  seem  to  have  little  difficulty  in  pro- 
ducing the  wonderful  results  just  referred  to. 


Fig.  16.     E.  A.  Apgar's  mechanical  finger. 

An  uncomplicated  form  which  has  been  very  success- 
fully used  by  its  inventor  is  that  shown  in   figure  16, 


MICROSCOPICAL    PRAXIS.  211 

having  been  devised  by  Mr.  Ellis  A.  Apgar,  who  at  my 
request  has  thus  described  it. 

It  is  constructed  out  of  the  stage  forceps,  the  forceps 
part  of  that  appliance  being  removed,  and  in  the  end  of 
the  arm  a  minute  hole  is  drilled  one-eighth  of  an  inch 
deep.  Into  this  is  inserted  a  bit  of  fine  spun-glass  or  a 
cat's  whisker,  and  kept  in  .place  by  wax.  A  wooden 
collar  is  fitted  to  the  front  of  the  objective,  and  in  the 
wood  is  bored  a  hole  of  the  proper  size  to  receive  the 
stem  of  the  forceps,  and  from  which  it  may  be  removed 
at  pleasure.  Thus  for  a  few  cents  an  almost  valueless 
implement  may  be  converted  into  one  that  is  often  use- 
ful, and  in  the  hands  of  an  expert,  one  that  is  capable 
of  performing  marvels  in  the  preparation  of  arranged 
diatoms. 

The  tube  through  which  the  stem  passes,  together 
with  the  socket-joint  give  the  operator  complete  con., 
trol  of  the  point  of  the  hair  or  of  the  glass  filament,  en- 
abling him  to  place  it  in  exact  focus  or  not,  as  he  may 
desire,  and  by  the  use  of  the  rack-and-pinion  move- 
ment of  the  body-tube  with  one  hand,  and  the  shifting 
of  the  stage  with  the  other,  the  minutest  diatoms  may 
be  selected  and  placed  wherever  desired;  delicate  ob- 
jects may  be  spread  out  and  arranged,  and  fine  parti- 
cles of  dust  may  be  removed  before  the  cover-glass  im- 
prisons them. 

The  slide  intended  to  receive  the  diatoms  or  the 
scales  in  patterns,  is  usually  slightly  coated  with  a  thin 
and  weak  solution  of  gum  arabic,  in  which  is  a  little 
acetic  acid,  and  allowed  to  dry.  The  object  being  de- 
posited in  the  proper  place  and  position,  the  slide  is 
gently  breathed  upon,  the  moisture  of  ihe  breath  soft- 
ening the  gum  sufficiently  to  allow  the  object  to  adhere 
the  thin  film  of  dried  mucilage  being  invisible  when  the 
Canada  balsam  is  added. 


212  MICROSCOPICAL    PRAXIS. 

The  Animalcule  Cage  or  Live-box. 

It  often  happens  that  the  microscopist  who  is  work- 
ing with  pond-life  needs  some  means  of  preserving  the 
life  of  the  aquatic  creatures  longer  than  is  possible  with 
the  ordinary  cement  cell  and  the  cover-glass.  There 
are  numerous  life-slides,  or  growing-cells,  well  adapted 
to  this  purpose,  but  the  animalcule  cage  of  the  optician 
has  the  additional  convenience  of  being  usable  as  a 
compressorium  as  well  as  a  life-slide.  It  is  somewhat 
inconvenient  however,  because  it  is  heavy,  difficult  to 
clean,  and  as  ordinarily  made,  almost  unusable  with 
high-power  objectives.  With  comparatively  large  ob-~ 
jects  and  low  powers  it  is  commendable. 

Mr.  E.  J.  Whitney  has  suggested  a  yery  cheap  and 
effectual  substitute  for  this  rather  expensive  piece  of 
apparatus,  which  he  makes  by  obtaining  at  the  hard- 
ware store,  a  full  set  of  about  a  dozen  ferrules  of  grad- 
uated sizes,  fitting  snugly  one  inside  the  other.  Take 
any  two  which  fit  well  together  and  cement  the  smaller 
one,  large  end  down,  to  the  centre  of  an  ordinary  glass 
slip.  To  the  top  of  the  ferrule  cement  one  of  the  thick- 
est cover-glasses  that  will  fit.  Now  take  another  cover- 
glass  that  fits  inside  the  larger  ferrule,  and  cement  it  to 
the  inside  at  the  top.  The  box  is  now  complete,  and 
all  that  remains  to  be  done  is  to  slip  the  larger  ferrule 
over  the  smaller. 


MICROSCOPICAL    PRAXIS.  213. 

Life-slides  or  Growing-cells. 

Some  of  these  numerous  devices  are  complicated  and 
expensive,  and  are  to  be  obtained  only  from  the  optic- 
ian. Others  are  simple,  easily  made  by  the  microsco- 
pist  himself,  and  are  as  praiseworthy  for  their  working 
qualities  as  those  offered  by  the  dealers.  The  purpose 
of  every  one  is  to  supply  enough  air  to  keep  the  im- 
prisoned plants  and  animals  in  good  condition,  so  that 
their  life-processes  may  be  performed  as  in  freedom. 

This  part  of  the  problem  is  not  so  difficult,  but  to 
supply  the  animals  with  the  proper  kind  and  amount  of 
food,  and  to  imitate  pretty  closely  the  environment 
which  they  must  have  or  suffer,  are  not  so  easy  of  ac- 
complishment. No  life-slide  has  successfully  solved 
these  parts  of  the  question.  All  forms  succeed  for  a 
time,  some  longer  than  others,  but  all  fail  within  a  very 
limited  period.  Those  animals  whose  life-history  is  be- 
gun and  finished  within  a  few  hours,  or  a  day  or  two  at 
most,  can  be  accommodated,  but  those  which  develop 
slowly  and  live  comparatively  long,  give  the  microsco- 
pist  a  difficult  and  troublesome  subject  to  consider. 

With  microscopic,  plants  the  question  is  no  more  easily 
settled.  They  die  as  readily  as  the  animals,  even  after 
they  have  received  the  most  careful  attention.  They 
refuse  to  continue  their  life-history.  Their  chlor- 
ophyll grains  fade,  and  slowly  group  themselves  to- 
gether near  the  centre  of  the  cell;  the  protoplastic  gran- 
ules increase  in  number  and  swarm  and  quiver  in  their 
ceaseless  pedetic  dance,  and  the  fungi  come  to  envelop 
the  whole  in  a  fluffy  winding  sheet,  which,  however  in- 
teresting at  the  proper  time  and  place,  is  not  accept- 
able in  one's  growing-slide. 

There  is  never  any  difficulty  in  cultivating  microscopic 
fungi  of  a  certain  kind.  They  cultivate  themselves,  and 


214  MICROSCOPICAL    PRAXIS. 

force  themselves  on  the  notice  of  the  microscopist  that 
has  them  always  with  him.  They  will  even  grow  and 
flourish  and  be  happy  in  the  horribly  astringent  solution 
within  the  alum-cell  of  the  oxy-hydrogen  microscope. 

The  simpler  the  life-slide  the  better  the  results,  and 
the  hastily-made  and  home-made  productions  are  often 
more  satisfactory  than  the  elegant  and  elaborate  con- 
trivances offered  by  the  dealers. 

For  my  own  purposes  I  take  for  covers  for  such  cells 
large  thin-glass  squares  only.  There  are  several  ad- 
vantages to  be  had  in  their  use  in  this  connection  over 
that  of  thin  circles,  one  being  the  facility  with  which  the 
water  supply  can  be  renewed.  By  carefully  adding, 
with  a  camel's-hair  brush,  a  drop  of  fresh  water  at  the 
corner  of  a  large  square  cover  projecting  beyond  the 
cement  cell,  the  fluid  will  usually  flow  under  so  gradu- 
ally that  the  object,  even  a  minute  Infusorian,  will  not 
be  moved  from  the  field,  the  inward  rush  of  the  current 
being  tempered  by  the  cement  ring.  This  supply  can 
be  easily  added  by  one  hand  holding  the  wet  brush 
while  the  eye  is  intent  at  the  ocular,  the  secret  of  suc- 
cess here  being  in  not  having  too  large  a  brush  and  in 
not  filling  it  too  full  of  water.  At  the  beginning  of 
daily  evening  work  the  brush  is  wetted  and  thrown  on 
the  table  to  become  thoroughly  moistened,  when  a  sin- 
gle dip  into  the  tumbler  of  water,  with  a  slight  shake 
to  prevent  dripping,  takes  up  enough,  although  some 
pressure  of  the  brush  against  the  slide  may  be  needed 
to  squeeze  out  a  small  drop.  It  is  better  to  make  sev- 
eral journeys  to  the  tumbler  than  to  lose  the  object.  A 
dipping  tube  adds  too  much  at  once,  and  cannot  be  so 
readily  controlled  as  a  brush. 

In  studying  the  morphology  of  minute  animal  organ- 
isms, I  use  only  a  shallow,  shellac  cell  with  about  one- 


MICROSCOPICAL    PRAXIS.  215 

fourth  of  the  ring  scraped  away  from  both  the  upper 
and  the  lower  margins,  thus  leaving  two  curved  sup- 
ports for  the  square  cover,  one  on  each  side.  The  dia- 


Fig.  17.     A  simple  life-slide. 

gram  shows  the  arrangement,  the  shaded  parts  repre- 
senting the  remnants  of  the  cell.      This  gives  the   en- 
closed   drop,   with    its  animal  life,  plenty  of  air,  and 
facilitates      the       application     of      the     wet      brush 
at  the  point  where  the  square  cover  projects  beyond  the 
lateral    cell-wall.      In    this   simple   affair    I    have  fre- 
quently kept  Infusoria  and  other  small  creatures  alive 
and  well  from  early  in  the  evening  until  after  midnight, 
and  when   compelled   to    leave   them     have     washed 
them  into  the  aquarium  in  as  good    condition   and    as 
lively  as  when  first  imprisoned.      Here  the  secret  of 
success  consists,  I  think,  in  leaving  enough  of  the  ce- 
ment ring  to  support  the  cover  properly  and   to   lessen 
the  force  of  the  inflowing  water-supply,  and  also  in  hav- 
ing the  cell  shallow  or  deep  according  as  the  animals 
are  microscopically  small  or  large.      Much  depends  on 
the  depth  of  the  cell  in  all  cases.    A  comparatively  large 
Infusorian,  a  Rotifer  or  a  Chatonotus  can  be  injuriously 
hampered  in  its  movements  and  in  the  proper  perform- 
ance of  its  functions,  by  a  cell  of  insufficient  depth,  and 
a  good  objective  can  as  the    reader  knows,  be  greatly 
hampered  in  its  functions  by  a  cell  of  too  great  depth. 
If  it  is  desired  to  convert  this  or  any  other  slide   of 
the  kind  into  a  growing-cell,   it   is  done   by   the  well- 
known    method    of   placing    the   slip   across   a   small 


2l6  MICROSCOPICAL    PRAXIS. 

saucerful  of  water  with  a  doubled  and  twisted  thread  of 
sewing-cotton  in  close  apposition  with  the  edge  of  the 
cover,  both  ends  of  the  thread  hanging  freely  in  the 
water.  The  liquid  will  flow  up  and  supply  that  lost 
by  evaporation,  provided  the  water  is  always  in  con- 
tact with  the  lower  surface  of  the  slide.  This  "dodge" 
is  successful  for  a  few  days,  but  it  always  ends  badly, 
as  the  salts  in  the  water  will  crystallize  at  the  cover- 
margins  and  cut  off  the  oxygen  supply. 

It  often  happens  that  the  conditions  and  the  environ- 
ment are  such  that  an  immense  number  of  minute 
Infusoria,  all  belonging  to  the  same  species,  are  sud- 
denly developed  in  the  aquarium,  infusion  or  macera- 
tion, and  it  becomes  interesting  to  isolate  a  few  to  study 
their  life-history.  Such  an  advent  of  Monads,  Heter- 
omita  and  other  similarly  minute  creatures  is  not  rare. 
For  the  study  of  such  truly  microscopic  objects  I  have 
devised  a  simple  life-slide,  describing  and  figuring  it  in 
"SCIENCE  GOSSIP"  for  January,  1894.  To  make  it,  ce- 
ment with  Canada  balsam  in  the  centre  of  a  slip  a  thin 
glass  disk  one-fourth  inch  or  less  in  diameter;  use  a 
one-sixteenth  inch  cover-glass  if  possible.  In  the 
letter  case,  then  take  a  glass,  or  zinc  or  other  kind  of 
ring  with  a  quarter-inch  aperture,  break  a  small  piece 
from  one  side,  and  fasten  this  broken  circular  band 
about  the  central  disc.  From  another  ring  with  a 
three-eighths  inch  or  larger  aperture,  break  a  piece  as 
before  and  cement  this  broken  band  around  the  inner 
ring  so  that  its  broken  part  shall  be  opposite  the  un- 
broken curve  of  the  latter,  and  with  a  thin  square  cover, 
the  cell  is  complete,  the  depth  depending  upon  the  dif- 
ference in  the  thickness  of  the  outer  rings  and  the  cen- 
tral circle. 

To  use,  place  on  the  central  disc  a  small  drop   of  the 


MICROSCOPICAL    PRAXIS.  2  17 

water  containing  the  organisms  to  be  kept  alive,  and 
over  it  arrange  the  square  cover,  taking  care  to  prevent 
the  water  from  overflowing  into  the  inner  annular  space. 
With  the  camel's-hair  brush  carefully,  and  in  small 
quantities,  add  fresh  water  at  the  top  or  the  side  of  the 
square,  never  at  the  bottom  or  near  the  opening  in  the 
outer  ring.  It  will  be  found  that  the  water  will  flow  be- 
tween the  square  and  the  upper  surface  of  the  exterior 
ring,  will  enter  through  the  break  in  the  latter,  partly 
filling  the  outer  annular  space,  and  by  capillary  attrac- 
tion will  occupy  a  part  of  the  vacancy  between  the 


Fig.  18.     A  simple  life-slide. 

cover  and  the  interior  ring,  (as  shown  by  the  diagonal 
lines  in  the  diagram,  Fig.  18),  but  unless  too  much  wa- 
ter is  used  or  it  is  supplied  in  too  great  quantities  at 
once,  it  will  not  pass  through  the  opening  in  the  inner 
ring,  thus  leaving  an  abundance  of  air  to  supply  the 
animals  under  observation.  The  imprisoned  air  at 
once  becomes  saturated  with  moisture,  as  is  evidenced 
by  the  fogginess  of  the  cover;  the  central  drop  can- 
not evaporate  and  the  external  water  will  not  come  in 
contact  with  it.  if  care  be  taken  in  filling  the  slide  and 
in  supplying  that  lost  by  evaporation.  To  admit  en- 
tirely fresh  air  the  water  can  be  drawn  off  by  bibulous 
paper,  or  allowed  to  evaporate,  without  in  any  way  dis- 
turbing the  central  drop.  The  reader  must  remember 
however,  that  the  cell  is  intended  for  only  the  smallest 
of  microscopic  animals,  with  which  it  is -entirely  suc- 
cessful. 


2l8  MICROSCOPICAL    PRAXIS. 

Logan's  Life-slide. 

Mr.  Logan's  device  consists  of  a  strip  of  wood  with  a 
central  perforation,  in  which  is  fitted  a  glass  cell 
formed  of  a  thick  platform  surrounded  by  a  deep 
groove.  The  object  is  placed  in  a  drop  of  water  on 
the  central  platform,  and  a  cover-glass  cemented  over 
the  whole  with  wax.  A  ring  of  sheet-wax  is  applied 
to  the  projecting  ring  around  the  groove,  and  the 
cover  is  fastened  down  by  running  a  warm  wire  around 
the  edge  to  melt  the  wax,  the  thickness  of  the  cell, 
that  is,  the  distance  of  the  cover-glass  above  the  cen- 
tral platform,  of  course  depending  upon  the  thickness 
of  the  wax  used.  The  air  confined  in  the  groove  by  the 
cover  will  be  enough  to  supply  the  microscopic 
creatures  for  a  long  time. 

Mr.  Logan's  slide  is  objectionable  because  it  is  so 
heavy,  the  central  disc  being  so  exceedingly  thick  that 
no  sub-stage  condenser  can  be  focussed  through  it,  and 
the  annular  depression  so  deep  that  the  glass  sides 
affect  the  light  in  an  undesirable  way.  For  compara- 
tively large  aquatic  objects  to  be  examined  with  a  low 
power  it  is  useful,  but  I  find  my  own  modification  of  it 
better  for  more  delicate  work. 

A  small  square,  cut  from  glass  of  any  desired  thick- 
ness, is  cemented  with  Canada  balsam  to  a  slip,  and 
surrounded  by  a  thick,  glass  or  zinc  ring  so  as  leave  a 
wide  space  between  these  parts.  On  the  ring  place  a 
ring  of  wax  and,  after  the  object  has  been  arranged  on 
the  central  square,  cover  the  whole  with  a  thin  cover 
and  cement  it  fast  by  running  a  warm  wire  around  the 
edge  to  melt  the  wax.  A  small  drop  of  water  may  be 
placed  in  the  annular  space  if  desired. 

A  diagram  of  this  slide  is  shown  in  Fig.  19.  The 
reader  of  course  understands  that  the  thickness  of  the 


MICROSCOPICAL    PRAXIS.  219 

slip  and  square,  and  the  depth  of  the  cell,  must  be  de- 
termined by  each  worker  according  to  his  needs.  For 
myself  I  have  them  as  thin  and  shallow  as  possible. 


Fig.  19.     A  simple  life-slide. 

The  secret  of  success  here  is  to  be  sure  that  the  joint 
between  the  ring  and  the  slip  is  air-tight,  and  to  secure 
the  cover  firmly,  using  an  abundance  of  wax. 

In  this  simple  contrivance  Hydra  viridis  has  lived  un- 
complainingly for  an  entire  month,  and  for  four  weeks 
a  Chironomus  larva  has  existed,  with  no  food  except 
what  may  have  accidentally  been  in  the  water,  and  no 
air  except  that  within  the  annular  space;  not  only  has 
the  same  larva  lived  in  that  confined  place,  but  it  there 
ceased  to  be  a  larva,  and  became  the  perfect  insect,  of 
course  soon  dying  for  the  want  of  air,  and  on  account 
of  its  inability  to  expand  and  to  dry  its  wings.  '  In  the 
same  form  of  life-slide,  with  an  abundance  of  algae  and 
of  infusorial  creatures,  the  rotifer  Furcularia  has  thrived 
and  built  her  mucilaginous  home  and  deposited  her 
eggs,  being  contented  there  for  a  whole  month  and  ap- 
parently willing  to  stay  another. 

For  showing  living  Infusoria,  Rotifers,  Chcetonoti, 
aquatic  worms  and  other  animals  at  microscopical  ex- 
hibitions, nothing  could  be  more  satisfactory  than  Mr. 
Logan's  slide.  With  it  the  writer  has  kept  a  quantity 
of  Turbellarian  worms  well  and  active  until  the  small 
hours  of  the  morning.  In  this  instance  the  slide  was 
prepared  for  an  exhibition,  so  hurriedly  and  so  late  that 
the  cement  was  not  dry  before  the  opening  hour 


22O  MICROSCOPICAL    PRAXIS. 

arrived,  but  an  external  application  of  the  thin  and 
rapidly-drying  Brown's  rubber-cement  made  all  tight, 
and  apparently  not  unpleasant  for  the  worms. 


Hitchcock's  Moist  Chamber. 

Mr.  Romyn  Hitchcock  has  used  and  recommended 
two  forms  of  simple  life-cells.  One  is  prepared  by  in- 
verting a  small  salt-cellar  in  a  dish  of  water  to  serve  as 
a  support  to  the  specimens,  a  tumbler  being  inverted 
over  the  whole.  The  objects  are  placed  on  glass  slips, 
made  by  cutting  an  ordinary  slide  across  the  middle, 
and  covered  with  thin  glass. 

The  second  plan  the  inventor  thinks  the  more  desir- 
able. --In  this  a  piece  of  glass  four  inches  square  is 
placed  on  a  support  so  that  it  is  about  on  a  level  with 
the  top  of  a  dish  to  hold  water,  an  ice-cream  saucer 
being  used  by  the  inventor.  A  piece  of  blotting-paper 
is  then  placed  on  the  glass,  and  the  edge  allowed  to  dip 
in  the  water.  Objects  to  be  examined  are  placed  on 
large  cover-glasses,  and  either  protected  with  a  smaller 
cover,  or  left  exposed.  These  cover-glasses  are  laid  on 
the  blotting-paper  with  watch  glasses  above  them.  A 
single  large  watch-glass  may  be  used,  or  a  number  of 
small  ones,  one  for  each  specimen.  Objects  can  be 
kept  fresh  and  moist  in  this  way,  with  far  less  trouble, 
Mr.  Hitchcock  says,  than  by  any  other  method  that  he 
has  tried. 


MICROSCOPICAL    PRAXIS.  221 

Beale's  Growing-cell. 

In  Fig.  20  is  shown  a  contrivance  almost  as  uncom- 


Fig.  20.     Beale's  growing-cell. 

plicated  as  the  preceding,  taken  from  Beale's  "How  to 
Work  with  the  Microscope,"  and  used  by  that  microsco- 
pist  with  much  success.  A  small  piece  of  glass  tube  is 
fixed  to  an  ordinary  slip  to  serve  as  a  reservoir  to 
supply  the  water.  It  is  covered  with  a  piece  of  thin 
glass,  a  small  opening  left  at  one  side  being  sufficiently 
large  to  allow  a  fine  thread  of  silk  or  cotton  to  conduct 
the  water  from  the  reservoir  to  the  specimen  placed  in 
the  centre  of  the  slide.  I  have  used  this  contrivance 
•with  considerable  satisfaction. 


H.  L.  Smith's  Growing-cell. 

This  is  a  very  efficient  cell  made  upon  the  old  princi- 
ple of  the  bird-fountain.  It  has  been  described  by  the 
author  as  follows: 

The  whole  slide  is  a  trifle  more  than  one-eighth  of 
an  inch  in  thickness,  and  consists  of  two  rectangular 
glass  plates,  three  by  two  inches,  and  about  one- 
twenty-fifth  of  an  inch  thick,  separated  by  strips  of 
the  same  thickness  cemented  to  the  interior  opposed 


222  MICROSCOPICAL    PRAXIS. 

faces,    as  shown    in    Fig.    21.      This  closed    cell,  ulti- 


Figure  21.     H.  L.  Smith's  growing-cell. 

mately  destined  to  be  filled  with  water,  is  not  of  such 
thickness  as  to  prevent  the  use  of  the  achromatic  con- 
denser, a  very  important  requisite.  The  upper  plate 
has  a  small  hole,  A,  drilled  through  it,  and  one  corner 
removed,  as  at  B.  A  small  strip  of  glass  cemented 
below  prevents  the  thin  glass  cover  over  the  object 
from  slipping.  Another  strip  is  cemented  on  the  lower 
side  of  the  cell  at  D,  but  not  extending  as  far  as  the 
removed  part  at  B.  The  object  of  this  is  to  prevent 
the  water  in  the  cell  from  being  removed  by  capillary 
attraction,  in  case  the  slide  in  the  neighborhood  of  B 
should  be  a  little  wetted.  This  strip  is  not,  however, 
absolutely  necessary. 

To  use  the  slide,  fill  the  space  between  the  two 
plates  with  clean  water,  introduced  at  B,  by  means  of  a 
pipette,  and  also  place  a  drop  on  A,  to  remove  the  air. 
The  object  being  put  on  the  top  of  the  slide  and  wetted, 
is  now  to  be  covered  with  a  large  square  of  thin  glass, 
C,  at  the  same  time  covering  the  hole  A.  The  slide  can 
now  be  placed  upright,  or  in  any  position,  and  no  water 
can  escape,  but  as  it  evaporates  from  under  the 
cover,  more  is  supplied  through  the  hole,  A,  and  from 
time  to  time  an  air  bubble  enters  at  *B;  thus  a  constant 


MICROSCOPICAL    PRAXIS.  223 

circulation  is  maintained.  A  cell  of  the  size  named  will 
need  replenishing  only  about  once  in  three  days,  and 
this  is  readily  effected  without  disturbing  the  object. 
I  have  been  enabled,  Prof.  Smith  continues,  to  make 
observations  by  means  of  this  slide,  which  it  would 
have  been  very  difficult,  if  not  impossible,  to  have  made 
without  it. 

The  cell  is  not  well-adapted  to  the  study  of  minute 
animals,  if  the  examination  is  prolonged  for  any  length 
of  time,  unless  the  aperture  in  the  cell  be  exceedingly 
small,  otherwise  the  active  creatures  will  surely  discover 
it  and  pass  through  it  into  the  reservoir.  They  will 
often  increase  greatly  there,  but  they  are  then  always 
beyond  the  reach  of  any  but  the  lowest-power  object- 
ives. For  algae,  desmids  or  other  still-life  the  slide  is 
commendable. 


Sternberg's  Culture-cell. 

A  simple  form  of  culture  cell  is  described  by  Dr.  Geo. 
M.  Sternberg,  as  a  member  of  the  Havana  Commission 
for  the  Investigation  of  Yellow  Fever.  It  is  made  by 
drilling  an  opening  about  one-fourth  inch  in  diameter 
through  the  centre  of  a  slip,  around  which  a  very  thin 
circle  of  cement  one-half  inch  in  diameter,  is  turned  on 
one  side  of  the  slip  and  a  thin  cover  attached  to  it  by 
gentle  pressure.  When  the  cement  is  thoroughly  dry, 
the  cell  is  ready  to  receive  the  drop  of  blood  or  other 


224  MICROSCOPICAL    PRAXIS. 

fluid  to  be  observed.     This  is   placed  in  the  bottom  of 
the  cell,  Fig.  22,  which    shows   the  slide  in  section,  and 

c 


Figure  22.     Sternberg's  culture-cell. 

flows  by  capillary  attraction  into  the  space  between  the 
thin  cover  and  the  slide  until  it  extends  to  the  circle  of 
cement.  Thus  there  is  a  thin  stratum  of  fluid  between 
the  points  b  and  c  which  may  readily  be  examined  by 
inverting  the  slide,  and  bringing  the  objective  to  any 
point  between  the  central  air-chamber  and  the  cement 
circle.  Finally,  the  cell  is  closed  by  fastening  a  large 
thin-glass  circle  to  the  opposite  surface. 


Strassburger's  Moist  Chamber. 

The  inventor  describes  this  to  consist  of  an  ordinary 
slide,  upon  which  is  placed  a  ring  of  pasteboard  moist- 
ened with  water.  The  object  which  is  to  be  observed 
and  kept  alive,  is  placed  in  a  drop  of  water  on  a  cover- 
glass  and  inverted  over  the  pasteboard  chamber,  the 
cover  being  made  to  adhere  to  the  cell  by  pressure. 
The  evaporation  of  the  water  is  greatly  retarded,  if  not 
entirely  prevented,  so  that  in  this  simple  manner  Prof. 
Strassburger  has  kept  Spirogyra  in  conjugation  alive  for 
several  days.  By  moistening  the  pasteboard  from  time 
to  time  the  cover  will  remain  attached  indefinitely. 


MICROSCOPICAL    PRAXIS.  225 

Deby's  Cell  for  Bacteria. 

This  is  a  three  by  one  inch  slip,  having  a  glass  ring 
with  ground  edges  cemented  to  it  to  form  a  cell.  A 
small  hole  is  bored  through  the  slip  inside  and  near  the 
edge  of  the  cell.  The  objects  are  placed-with  a  very 
minute  drop  of  water  on  a  thin  cover,  which  is  inverted, 
and  attached  to  the  top  of  the  cell  by  a  little  lard.  The 
slip  is  then  laid  upon  another  of  the  same  size,  but  not 
perforated,  and  a  couple  of  India-rubber  bands  are 
passed  over  the  ends.  One  end  of  this  arrangement  is 
then  placed  in  a  little  water,  which,  by  capillary  action, 
will  occupy  the  space  between  the  two  slips,  and  by 
evaporation  will  rise  into  the  cell  and  prevent 
the  drying  up  of  the  minute  drop  on  the  cover. 
By  this  contrivance  a  drop  of  water  no  larger  than  a 
pin's  head  can  be  kept  of  nearly  the  same  size  for 
weeks  together,  and  the  development  of  bacteria,  or 
other  minute  organisms,  retained  constantly  under  ob- 
servation. 

M.  Deby  has  also  described  what  he  considers  to  be 
a  simplification  of  the  foregoing.  Take  a  slip  and 
make  in  its  centre  a  circular  opening  ^  inch  in  diam- 
eter; lay  it  upon  a  slide  not  perforated,  and  bind  the 
two  together  by  rubber  rings.  Grease  the  upper  slip 
for  a  short  distance  around  the  opening,  arrange  the 
object  on  a  thin  cover,  adding  to  it  another  cover  % 
inch  in  diameter,  which  will  adhere  by  capillary  attrac- 
tion, and  invert  the  whole  over  the  central  aperture. 
When  not  under  observation  place  in  a  shallow  vessel 
of  water,  as  in  the  original. 


16 


226 


MICROSCOPICAL    PRAXIS. 


Pagan's  Growing-slide. 

The  Rev.  A.  Pagan  has  devised  the  following  for 
the  study  of  rotifers,  algae,  and  other  microscopical  or- 
ganisms needing  a  frequent  change  of  water.  He  states 
that  the  results  obtained  with  it  were  remarkable.  It 
may  be  kept  constantly  on  the  microscope  stage,  if  de- 
sired. 


Fig.  23,  Pagan's  life-slide. 

It  consists,  as  shown  in  Fig.  23,  of  a  slip  A  slightly 
longer  than  the  stage,  so  that  it  shall  project  a  little  at 
both  ends.  On  it  is  placed  a  piece  of  blotting-paper 
which  leaves  only  the  margins  of  the  slip  free,  and  a 
hole  is  cut  in  the  centre  of  the  paper,  B  C,  and  at  one 
end  is  the  triangular  prolongation,  B ',  which  is  bent 
downward  close  to  the  slide.  Water  is  then  drawn  from 
the  vessel,  Z>,  by  means  of  the  capillary  tube,  E,  and 
drops  on  the  blotting  paper.  Tne  tube  should  be  only 
wide  enough  to  allow  one  drop  to  fall  every  twenty  sec- 
onds, the  water  being  drained  off  by  the  triangular  pro- 
longation. An  inverted  flask,  F,  may  be  filled  with  wa- 
ter, and  so  placed  that  its  mouth  shall  just  touch  the 
surface  of  the  fluid  in  the  tumbler,  and  keep  the  level  of 
the  water  constant,  thus  ensuring  the  regular  escape  of 
drops  from  the  capillary  tube;  or  to  simplify  the  appar- 
atus, this  vessel  may  well  be  omitted. 


MICROSCOPICAL    PRAXIS.  227 

Slack's  Tubular  Live-box. 

Mr.  J.  H.  Slack  makes  a  tubular  cell  for  the  examina- 
tion of  the  mouth-organs  of  insects.  He  takes  a  small 
homeopathic  phial  about  half  an  inch  long,  and  a  quarter 
of  an  inch  wide  at  the  mouth.  This  is  inserted  into  a 
hole  cut  in  a  wooden  slip,  the  rim  of  the  bottle  prevent- 
ing it  from  falling  through.  Another  wooden  slip  has  a 
hole  through  it  of  larger  diameter,  and  on  the  top  is 
cemented  by  shellac  a  thin  glass  cover.  This  slip  is 
laid  on  the  other,  the  glass  cover  forming  a  lid  which 
closes  the  bottle,  the  whole  being  held  in  position  by  a 
rubber  band.  A  little  cotton  wool  is  put  into  the  bot- 
tom of  the  bottle  to  suit  it  to  the  length  of  the  fly,  which 
must  be  inserted  mouth  upward,  and  kept  moderately 
near  the  cover-glass,  upon  which  a  drop  of  syrup  is 
placed.  Flies  will  readily  feed  in  this  position,  and  they 
are  sufficiently  limited  in  their  power  of  lateral  motion 
to  be  easily  kept  in  the  field  of  the  one-inch  or  of  the 
one  and  one-half  inch  objective. 

This  might  be  modified  by  cutting  off  the  bottom  of 
the  bottle  so  as  to  make  a  tube  of  it.  The  fly  could 
then  be  introduced,  after  the  cover-glass  had  been  ar- 
ranged, and  in  the  proper  position  with  its  mouth  up- 
permost. It  could  thus  be  gently  forced  to  ascend  to 
the  desired  place  by  forcing  the  cotton  wool  in  behind 
it. 

There  are  many  other  forms  of  life-slides  or  of  grow- 
ing-cells, but  these  will  be  enough  to  give  the  reader  an 
idea  of  what  is  needed,  and  of  the  simplest  way  to  ac- 
complish the  desired  results.  The  less  complicated  the 
apparatus,  the  better  it  will  be. 

THE    END. 


228  MICROSCOPICAL    PRAXIS. 


Index. 


A. 

Page 

Abbe  apertometer,     -  120 

Abbe  camera,    -  205 

Aberration,  chromatic,       -  104 

"          spherical,  107 

Acme  lamp,  31,  35,  36,  38,  146 

Acme  stand,  Queen  &  Co.'s,  -                                      22 

Adjusting  the  objective,    -  93 

"          by  color,  -    98,  99 

"          by  tube-length,  101 

"          over  Pleurosigma,  -                                      99 

"            "      Podura,    -  -    96,  98 

Adjustment,  change  of  by  change  of  eye-piece,         149 

"          coarse,      -  9,  18 

"          collar  and  value  of  micrometer-spaces,         64 

fine,  -      9,  20 

"            "    position  of,  21 

"            "          "     of  Queen's  Acme,     -  22 

Air  bubbles,  76 

"            to  make,  77 

"            to  recognize,  7  8 

Air,  refractive  index  of,     -  113 

Amphipleura  pellucida,      -  i32 

Amplifier,  the,  *93 

Analyser,  the,    -  *9& 

Angle  of  field,  122 


MICROSCOPICAL    PRAXIS.  •                           229 

Page 

Animalcule  cage,  212 

Apertometer,     -  120 

Aperture,  angular,     -  112 

"              "       by  image-forming  rays,  121 

"              "        "       "            "            "  to  measure,  1 23 

"        of  certain  objectives,  -                  126 

"       to  measure,    -  117 

numerical,  112 

"               "         and  resolving  power,  -  131 

"               "          to  measure,  127 

"       table  of,      -  114 

"       to  use,         -.  113 

Apgar's,  E.  A.,  mechanical,  finger,     -  209 

Ashe,  A.,  cited.  146,  147 

B. 

Bacillus  tuberculosis,  153 

Bacteria,  Deby's  cell  for,  -  225 

Balsam,  Canada,  refractive  index,  -  113 

"  "  diatoms  in,  -  TOO 

Bausch's,  Edward,  adjustment-method,  -  99 

Bausch  &  Lomb's  camera  lucida,  -  208 

Bausch,  Edward,  cited,  -  99,  104 

Beale's  adjustment-method,  -  95 

Beale's  growing-cell,  221 

Beale,  L.  S.,  cited,  -  *  95 

Beale's  reflector,  202,  203 

Beetle,  Gyrinus,  key  to  species,  6 

Beck,  R.  and  J.,  cited,  -  -  17,  96 

Black-ground  illumination,  117,  178 

"  "  illuminators,  181 

Blackham,  J.  G.,  cited,  -  123 

Blow-fly,  proboscis  of,  -  128 

Body  of  stand,  8 

Body-tube,  -  -  -  n 


230  MICROSCOPICAL    PRAXIS. 

Page 

Body-tube,  cloth-lined,  12 

"      "      standard  length,  35 

Brownian  movement,  76,  79 

Bulloch's  apparatus  to  measure  power  of  eye-pieces,  54 

Bulloch,  W.  H  ,  cited,  23,  54 

Bull's-eye,  position  of,       -  35 

"       "     to  measure  foci  of,  35 

%      C. 

Camera  lucida,  -  61,  200 

Abbe's,  205 

"          "        drawing-desk  for,  207 

Bausch  &  Lomb's  208 

"           "        Grunow's,  -  208 

measuring  by,     -  61 
measuring  thickness  of  cover-glass  by,  85 

Wollaston's,  204 

Cedar  oil,  refractive  index,  113 

"        "  to  free  front  lens  from,  163 

Cell  for  monochromatic  light,    -  42 

Central  illumination  by  air  bubbles,  78 

Centre,  importance  of  the,  44 

Centring  illumination,  Pennock's  method,  44 

Cholera  spirillum,      -  153 

Chromatic  aberration,        -  104 

tests  for,  -104,  108 

•Closed  point,     -  -     92,  94 

Collar  adjustment,     -  92 

•Color,  adjusting  by,  98 

Color  corrections,  table  of,  105 

Coma,        -  94 

Condenser,  Abbe's,    -  175 

"          achromatic,  175 

11           Bausch  &  Lomb's,  -  17? 

"           central  illumination  with,          -  180 


MICROSCOPICAL    PRAXIS.  231 

Page 

Condenser,  diaphragm  carrier  of,       -                  .  177 

diaphragms  of,  i7I>  I76 

immersion,       -  ^ 

sub-stage,  T69 

to  focus,                                       -         _  J74 

to  modify  illumination  with,    -  172 

objective  used  as,    -  ^5 

Powell  &  Lealand's,  I76 

"           Watson  &  Son's,      -                   -         .  I76 

Concave  and  convex  surfaces,  aspect  of,  -  -      -  80 

Correction,  over  and  under,       -  -         -  98,  104 

Cover-glass,  correcting  objective  for,  91 

"      forceps,  89 

"      on    mounted    objects,    to    estimate 

thickness,       -                                      .  86,  87 

"      optical  effect  of,                    '  ..  ,     .  gi 

"      thickness,  collar  graduations  and,  -  103 

Cox,  J.  D.,  cited,       -                                              -  185 

Culture-cell,  Sternberg's,                             ,.         .  223 

Czapski's    method    of    measuring     covers     on 

mounted  objects,     -                  ...  86 
Czapski,  S.,  cited,                                            . '       -82,  86 

D. 

Dallmger,  W.  H.  cited,      -  -  42,  171 

Daylight,  mirrors  and.       -                            -         -  40 

use  of,                                              .         .  38 

when  commendable  for  microscopical 

work,                                                           .  4o 

Definition,                                       .         .         .         .  I28 

limit  of  power  for  best,     -  157 

tests  for,  -  i28 

Deby's  cell  for  bacteria,                        -         -         -  225 

Devron,  G.,  cited,      -  193 

Desk,  drawing,  for  Abbe  camera  lucida,    -  207 

Diameters  defined,     -                                      -  „  4 


232  MICROSCOPICAL    PRAXIS. 

Page 

Diaphragm,        -  46 

"             action  of,  48 

defined,  10 

"            displacement  of  in  body-tube,  73 

of  condenser,  171,  176 

"             position  of,     -  46, 47, 48 

Diatom,  balsam  mounted,  100,  131 

"        dry  mounted,  100,  131 

"       secondary  structure,       -  152 

"        striae  and  Nobert's  lines,  table  of,  136 

"       tertiary  structure,  154 

"        test  plates,     -  132 

tv        usefulness  of,  152 

"        variation  of  striae,  131 

Douglas's  device  for  handling  thin  covers,  89 

Drawing  the  object,  200 

Drawings,  enlarged,  to  make,    -  205 

Draw-tube,  8,  15,  17 

"         "      for  each  inch  extended,   increase  of 

power,    -  1 8 

"         "      increase  of  power  by,       -  17 

"         "      low-power  objectives  in,  16 

"         "      to  extend,        -  16 

Dust,  effect  of,  75 

u     to  dispel  from  eye-piece,  -  56 

E. 

Enlargement  of  drawing,  to  ascertain,  67 

Erector,  the,      -  194 

Swell's  micrometers,  58 

Eye,  effect  of  artificial  light  on,  39 

Eye-lens  of  ocular,  to  measure  power  of,  -  51,  52 

Eye-piece,  -       8,  49 

"       "    anterior  focus  of,  143 

"       "    compensating,  150 


MICROSCOPICAL    PRAXIS.  233 

Page 

Eye-piece,  diaphragm,  use  of  for  micrometer,    -  50 

"       "    for  micrometer,  best,  -  65 

"       "    function  of,  49 

"       "    Huyghenian,        -  5° 

"       "   micrometer,  62 
"       "             "          to  learn  value  of  spaces  of          63 

"       "    negative,  49 

"       "   parfocal,     -  15 l 

"       "    particles  on,  76 

"       "    positive,       -  5° 

"       "   power  of,    -  55 
"       "       "       "  affects  value  of  micrometer 

spaces,  64 

"       "    rating  of,     -  •       -  53 

"       "   specks  on,  -  7^ 

Eyes,  keep  both  open,       -  73 

Eye-shades,     -  -  74 

Eye-training,     -  13°*  T52 

F.  '.* 

Fasoldt's  test-plate,  -  -     43,  58 

Field,  actual,    -  51 

"           "        of  certain  objectives,  52 

"           u         to  measure,  51 

angle  of,  I22 

"         apparent,       -  51 

""             "          to  measure,     -  51 

cause  of  size  of,    -  5° 

centre  of  for  measuring  objects,  109 

flatness  of,    -  I09 

sweeping  the,  27 

"         to.  illuminate,  7T 

of  view,  9 

Filter  paper,  Japanese, 

Finders,     -  l68 


234  MICROSCOPICAL    PRAXIS. 

Page 

Fine  adjustment,        -  -      9,  2o 

used  as  micrometer  gauge,  84 

Finger  mechanical,    -  -         209 

Focus,       -  9 

"      objective,  167 

•'      of  concave  mirror,  -  33 

"      of  objective,  to  measure,  158 

"      posterior  principal,  -  142 

table  of,  145 

"         Queen's  2  in.  objective,       146 

"  Zeiss's  A*  objective,  -         146 

"      within  and  without  the,  -  94 

Foot  of  stand,  .             8 

Forceps,  mounting,  Chase's,  89 

stage,  -  209 

Frey,  H.,  cited,  78 

Frustulia  Saxonica,    -  132 

G. 

Gage,  A.  P.,  cited,  I97 
Gage,  S.  H.,  cited,     -                            56,  77,  88,  102,  206 
Glass,  blue,        -                                                       -  43 
"           "  for  condenser,  -                                              173 
"      cover,      -  8 1 
"      crown,  refractive  index  of,       -                           113 
"      thin,  circles  and  squares  how  cut,    -  82 
"         "     device  for  handling,  89 
"         "     Hanaman's  method  of  storing,                     83 
"     how  made,        -                                                82 
"     thickness  used  by  the  various  opticians,  88 
"         "     to  clean,  -                                                           83 
"     to  measure  thickness  by  camera  lucidia,      85 
"         "  micrometer-gauge  84 
"                 "        on  mounted  ob- 
jects,                   86,  87 


MICROSCOPICAL    PRAXIS.  235 

Pag« 

Glycerine,  refractive  index,  113 
Graduations  of  adjustment  collar   and    cover- 
thickness,  103 
Griffith-club  stand,    •  25 
Griffith,  E.  H.,  cited,  -  25,  188 
Grooves,  optical  appearances  of,  81 
Growing-cells,  209,  213 
"         •'       Beale's  221 
"         "       Pagan's  226 
"         "       simple,         -   f  214 
"         "      Smith's  222 
Grunow's  camera  lucida,  -  208 
Gymnastics,  microscopical,  43 
Gyrinus,  key  to  species,    -  6 

H. 

Hall,  J.  B.,  cited,  74 

Hanaman,  C.  E.,  cited,  83 

Heads,  milled,  -  n 

Hitchcock,  R.,  cited,  220 

I. 

Iceland  spar,      -  197 

Illumination  affected  by  position  of  mirror,        -  33 

"            black-ground,  i77>  J78 

central  by  air  bubbles,  -  78 

"       with  condenser,  180 

"            diabolical,      -  78 

"             general,  of  work  table,  -  -     .    *  4° 

"             oblique,  78 

"                '"        with  condenser,  177 

"            proper,  of  microscope,  -  31 

"             to  make  central,     -  44 

"             white  cloud,  4° 

Illuminator,  Stratton's,  3r>  35»  38 

"            vertical,  l89 


236  MICROSCOPICAL    PRAXIS. 

Page 

Illuminators,  black-ground,        -  181 

supra-stage,  187 

Image,  effect  of  daylight  on,      -  39 

Image -forming  rays,  -         .         I2i 

Immersion-fluid,  to  apply,  -                            !6i 

"      to  measure  index  of,                           164 

"     to  remove,  -         .         163 

Immersion-media,  homogeneous,  -         -                  164 

Immersion  paraboloid,        -  -                            ^2 

Index,  refractive,  of  various  substances,   -         -         113 

Indicator  for  ocular,  ...           c6 

Instrument,  to  care  for  the,  -         -           67 

L. 

Lamp,  Acme,  3Ij  35>  3s 

"      best  position  for,      -  34 

"      flame,  edge  of,  ...           38 

"      position  of,  .     34>  7! 

"     Str*tton,  .        31,35,38 

with  circular  or  flat  wick,  41 

Length  of  body-tube,  standard,  -             4 

Lens,  convex,  as  camera  lucida,  -          41 

"      hemispherical,  i33)  186 

"      sP°t,  183 

Lepisma,  scales  of,  -         .         I3^ 

Lieberkuhn,  the,  -         .         187 

Life-slides,  -         .         .         213 

"      Logan's,  2l8 

"      simple,  219 

Light,  artificial,  advantage  of,  -  39 

"      for  drawing,  to  modify,    -  200 

11      from  oil  or  gas,  41 

"      lamp,  to  make  rays  parallel,  33,  35 

"      monochromatic,       *-  -     41,  42 

cell  for,  -                             42 


MICROSCOPICAL    PRAXIS.  237 

Page 

Light,  monochromatic,  copper  sulphate  solution  for,     41 
"  "  potassium    bichromate 

solution  for,    .-  43 
"      oblique,                                                  23,  29,  78,  133 

"      polarised,  196 

"   '  reflected,  45 

"      transmitted,     -  45 

"      when  more  is  needed  by  objective,  -  180 

Lighting,  proper,  for  microscope,      -  31 

Lines,  ruled,  Fasoldt's,      -  58 
"          "      Nobert's,                                                  43,  58 

"          "      visibility  of,   -  131 

"      to  inch,  greatest  number  seen,  58 

"       "      "    million,  58 

Live-box,  209 

"       "    Slack's  tubular,  227 

Lubricating  working-parts,  76 

M. 

Magnifying  power,    -  141 

"      high,  -  .       159 

"      initial,  -  155 

"               "      limit  of  for  best  definition,     -  157 

"               "      limit  of  useful,       -  159 

Maltwood  finder,       -  168 

Measuring  by  camera  lucida,     -  61 

"          objects,  centre  of  field  for,  no 

Mechanical  finger,     -  209 

Mica  covers,      -  81 

Micrometer,       -  58 

"          Ewell's,  58 

"          eye-piece,  62 

"            "       "     proper  ruling  of,       -  65 

"            "       "     to  learn  value  of  spaces,   -  63 

Fasoldt's, 58 


238  MICROSCOPICAL    PRAXIS. 

Micrometer  guage,                       ...  g4 

"      Bausch  &  Lomb's,  85 

"      fine-adjustment  used  as,  84 

Rogers's,  .           5g 

spaces,  estimating  parts  of,  -                    60 

recording  value  of.  -         -           65 

stage,      -  59 

standard  for,  -  .                    62 

Micron,  value  of,  62 

Microscope,  to  illuminate,  7o 

to  remove  from  case,  68 

to  return  to  case,  .           7^ 

Microscopical  gymnastics, 

Microscopist,  amateur,  influence  of,  ^3 

Microscopists,  American  Society  of,  -  62 

Mirror,  I0 
Mirror-bar, 

"      best  position  of,  ,. 

"      concave,  action  of,  ^2 

axis  of,  34 

distance  from  lamp,  proper,      -  36 

"      object,  37 

size  of,     -  -           31 

to  learn  diameter  of  its  hollow  sphere,  33 

to  measure  focus  of,  33 

"      plane,  action  of,     -  32 

"      reflector,  a  simple,  203 

"      size  of  concave,      -  3! 

Mirror,  to  manipulate  the,  7T 

"     use  of  the,       -  3I 

Moist  chamber,  Hitchcock's,     -  220 

Strasburger's,  -         224 

Moller's  test-plate,  diatoms  on,  -         134 

Moore,  A.  Y.,  cited,  .    98,  127,  185 


MICROSCOPICAL    PRAXIS.  239 

N. 

Naegeli  &  Schwendener,  cited,  105,  in 
Nelson,  E.  M.,  cited,                                         128,  155,  157 

Nicol  prisms,     -  197 

Nobert's  lines,  -  43,  58,  135 

"          "      and  diatom  striae,  table  of,  •  136 

Nose-piece,  safety,     -  14 

O. 

Object,  how  image  should  appear,     -  93 

"       opaque,  under  Woodward  prism,  -  785 

"       to  place  small  in  field,  -  81 

Objectives,  actual  field  of  certain,     -  52 

"            adjustable  and  non-adjustable,  91 

"            adjusting  by  cover  imperfections,  -  95 
"            adjustment,  change  of  with  change 

of  eye-piece,    -  149 

"            aperture  of  certain,  126 

"            as  an  erector,  195 

"            corrected  for  certain  tube-length,  4 

"            correcting  for  cover,        -  91 

"            defined,  8 

"            defining  power  of,  -  128 

"         .  for  cover,  correcting,       -    •  91 

"            homogeneous,  Stevenson  and,  113,  160 

"           how  rated,        -  138 

"            immersion,       -  160 

"                    "         to  clean,  -  161 

"            in  draw-tube,  low-power,  16 

"            low-power,  to  illuminate,  -    72,73 

"           needs  more  light,  when,  -  180 

"            optical  centre,  142 
"            position  of  posterior  principal  focus 

of  certain,       -  145 


240  MICROSCOPICAL    PRAXIS. 

Page 

Objectives,  posterior  principal  focus,  142,  143 

"                  "                 "             "      to  obtain,  144 

"            Powell  &  Lealand's  -^,  159 

"           specks  on,  seen  in  field,  -  76 

the,  90 

to  adjust  by  coma,  94 

"            "        "       "    cover  imperfections,    -  95 

"            to  attach  to  tube,    -  69 

Tolles's  ^,  159 

"            to  measure  focus,    -  158 

"           to  obtain  posterior  focus,  144 

"           used  as  condenser,  176 

u            Queen  &  Co.'s  2  inch,      -  146 

"            Zeiss's  A*,       -  -      52,  72,  146 

Oblique  light,  23,  29,  78,  133 

Ocular,      -  -      8,  49 

Oil  bubbles,        -  -    76,  77 

"        to  make  and  recognize,  -  78 

Opaque  objects  under  Woodward  prism,   -  185 

Open  point,        -  -    92,  94 

Optical  centre  of  objective,        -  142 

Over  and  under  correction,        -  -  98,  104 


P. 

Pagan's  growing-slide,       -  226 

Paper,  Japanese  filter,  56 

Parabolic  speculum,  188 

Paraboloid,  182 

Pedesis,     -  79 

Penetration,      -  137 

Pennock,  Edward,  cited,    -  -        44,  72,  101,  104,  151 

Pennock's  method  of  centring  the  illumination,  44 

Peragallo,  H.,  cited,  171 


MICROSCOPICAL    PRAXIS.  241 

Page 

Pillars  of  stand,  8 

Pinion  of  stand,  9 

Plaster-of-Paris  plate,  40 

Pleurosigma  angulatum,                      -     99,  101,  128,  132 

"            formosa,  128 

Pocket-lens,  chromatic  and  spherical  abberrations,   106 

"             combination,  i 

"             convenient  power  for,  -  2 

"             diaphragm  of,        -  i 

"             simple,  i 

"             test  for,  5 

to  measure  focus,  2 

"                         "        magnifying  power,       -  3 

Podura,      -  95 

"       scale,  appearance  of,      -  -    96,  97 

Polariscope,       -  196 

Polariser,  198 

Posterior  focus  of  objective,      -  142,  143 

"             "       "         "         to  ascertain,    -  144 

Power,  increase  of  by  draw-tube,       -.-  •  17 

"      initial  magnifying,  -       •  155 

"      magnifying,    -  141 

high,     -  159 

"  "  limit  of  for  best  definition,    -         157 

"               "              "      "  useful,      -  159 

u  "  table  of  increase  for  each  inch 

of  draw-tube,      -  18 

"        resolving,      -  129 

"               "         to  increase,    -  136 

Prism,  Nicol,     -  196 

"      right-angled  as  erector,  195 

"      Woodward,     -  184 

"               "            opaque  objects  under,  185 

Proboscis  of  blow-fly,  128 


242  MICROSCOPICAL    PRAXIS. 

Page 

R. 

Rack  of  stand,  -  9 

Ray,  ordinary  and  extraordinary,      -  198 

Reflector,  a  simple,  -  203 

"          neutral  tint,       -  202 

"               "         "     disadvantage  of,      -  203 

"          thin-glass,  201 

Refractive  index  of  immersion  fluids,  to  measure,         164 

"              "      Smith's  test  for,       -  165 

Resolving  power,       -  129 

"              "      to  increase,     -  136 

Roger's  micrometers,  58 

S. 

Scale  and  vernier,      -  15 

Scales,  Lepisma,  135 

"      use  of  Podura,  96 

Schultz,  A.,  cited,      -  191 

Screw,  Butterfield,     -  13 

"      fine-adjustment,  value  of  graduations,     -  22 

"      society,  -  12 
Selenite,    -                                                                  196,  199 

Slack's  tubular  live-box,    -  227 

Slide,  10 

Slip,  10 
Smith,  H.  L.,  cited,    -                                               165,  189 

Smith's  growing-cell,  222 

Smith's  test  for  refractive  index,        -  165 

Soap  and  water,  Brownian  motion  of,  80 

Spherical  aberration,  107 

Spaces  of  micrometer,  number,  60 

Speculum,  parabolic,  JfS8 

Spot  lens,  183 
Spring  clips,      -                                                           -      10,  24 

"     of  Griffith-Club  stand,     -  25 


MICROSCOPICAL    PRAXIS.  243 

Page 

Stage,  -      9,  25 

"      mechanical,      -  25 

"      micrometer,     -  59 

"                 "         to  use,   -  60 

"      safety,      -  27 

"  '        4<         Stewart's,    -  28 

"       special,  25 

Stand  and  its  parts,  7 

"     Griffith-Club,    •  24 

"     to  prepare  for  use,    -  69 

"     without  coarse  adjustment,  -                              19 

Star,  artificial,  -  106,  107 

Sternberg's  culture-cell,     -  223 

Stevenson,  J.  W.,  and  homogeneous  objectives,    1 13,  160 

Stop  defined,     -  10 

Strasburger's  moist  chamber,     -  224 

Stratton  illuminator,  31,  35,  38,  146 

Styrax,  refractive  index  of,  113 

Sub-stage,  n 

Suffolk,  T.,  cited,       -  201 

Sunlight,  direct,  when  used,       -  40 

Surirella  gemma,        -  132 

T. 

Table,  aperture,  114 

"             "         of  certain  objectives,  126 

"      numerical  aperture,  to  use,  -                             113 

"      the  work,  68 

"      tube-length,    -  103 

Test-plate,  Abbe,       -  107 

MOller's  and  Thum's,  -                             134 

"            of  diatoms,      -  132 

Thickness  of  cover  glass,  -  84 

Thin  glass,  81 

Times  and  diameters  defined,    -  4 


244  MICROSCOPICAL    PRAXIS. 

Page 

Triplet,  achromatic,  -  i 

"  "  useful  power  for,  2 

Tube-length,  adjusting  objective  by,  101 

"  of  various  opticians,  -  102 

"  optical,  143 

"  "  to  measure,  146,148 

u  standard,  -  4 

Tubular  live-box,  Slack's,  227 

V. 

Vernier,  scale  and,     -  5 

Vertical  Illuminator,  189 

Vision,  arbitrary  distance  for  distinct,  3 

W. 

Water,  refractive  index,     -  113 
Wead's  method  of  measuring  covers  on  mounted 

objects,  87 

Wenham's  method  of  adjusting  objective,  94 

Wenham,  W.  H.,  cited,       -  94 

Whirligig  beetles,  key  to  species,       -  6 

Whitney,  E.  J.,  cited,  212 

Wollaston  camera  lucida,  204 

Working  distance,     -  -     i,  138 

"               "         to  measure,    -  139 

Woodward,  J.  J.,  cited,      -  -     43,  65 

Woodward  prism,       -  184 

Z. 

Zeiss's  A*  objective,  field  of,  -  52 

"  "  "  posterior  focus,  146 

"  "  "  to  illuminate,  -  72 

Zentmayer,  J.,  cited,  23 


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